Temperatures below that of freezing mercury are usually mea. sured by a thermometric tube containing alcohol; and temperatures higher than that of boiling mercury are measured by the expansion of air or of metals, as above described, or by instruments called pyrometers, which are specially constructed for this purpose.

While "thermometer " implies the measurer of heat, "pyrometer" implies the measurer of fire or temperatures above the range of the mercurial thermometer. These instruments depend for their action on the expansion of metals or of air. The results are not so accurate as those obtained within the range of the mercurial ther

mometer.

Formerly much importance was laid on the use of Wedgwood's pyrometer for high temperatures. He employed small cylinders of baked clay, which underwent contraction by heat, and the degree of contraction was measured by placing the cylinder, after heating it, in a groove in a brass plate, graduated. This groove was gradually reduced in width from the beginning of the scale, and the point which the cylinder reached and fitted, after it had been heated, was supposed to measure the degree of heat. This method has been laid aside as very inaccurate. There was no reliable standard with which it could be compared, and it was found that a long-continued low heat had the same effect on the clay-cylinder as a high temperature of short duration. The zero of this pyrometer was placed at a full red-heat, i.e., about 1000° F., and it went up to 240°, corresponding to 32,277° F.

718. It is very interesting, while considering the vast number and importance of the phenomena produced by heat, to observe the degrees in the general scale of temperature at which certain changes take place. In the following table a selection of the facts connected with temperature has been made in some cases from actual observations, and in others from reliable authorities. They are all expressed on Fahrenheit's scale, the only thermometer used in this work.

to any hour at which it may be desirable to record temperature. When the hand arrives at the hour, the thermometer is turned over by a simple clock movement; the thread of mercury breaks off and falls into a graduated tube where the degree can be read off.

* From the Greek, Tup, fire, and μéтpov, measure.

506

Table of High and Low Temperatures.

Table of facts connected with the influence of heat corresponding to certain temperatures.

Degrees below zero F.

Greatest artificial cold produced by nitrous oxide
and bisulphide of carbon in vacuo (Natterer)
Greatest cold from a bath of carbonic acid and

ether in vacuo* (Faraday)

Liquefied nitrous oxide freezes

Liquefied sulphurous acid freezes.

Greatest natural cold (Siberia-Erman)

Liquefied carbonic acid freezes

Estimated temperature of planetary space

(Fourier).

Mercury freezes

Mixture of equal parts of sal amraoniac and ice
(Fahrenheit's zero)

Degrees above zero F.

Air on the summit of Mont Blanc (15,781 feet)
February, 1876, 3 P.M.

Ice melts (zero of Centigrade and Réaumur)
Animal heat-the human body (Blood-heat)

Highest natural temperature observed in India
Steamship engine-room, West Indies

Alcohol boils

Water boils.

Tin melts

Bismuth melts

Lead melts

Mercury boils

Black heat

Zinc melts

Antimony melts

Red heat, visible in the dark.

visible in daylight

Heat of a common fire

Bright red heat

Silver melts.

Gold melts

* At this temperature Faraday observed that the alcohol used, became

thick, like castor oil, and therefore ceased to measure lower degrees with

Passage of Heat into Light.

507

719. All solid bodies are considered to become visibly red or incandescent at or about 1000°, hence called a red heat, although observers are not agreed as to the precise temperature at which this takes place. A red heat visible in daylight, is, according to some authorities, 977°; others place it at 1000°, and others, again, at 1100°. This last statement is more in accordance with the writer's observations. Not only solid substances, but probably all liquids, not entirely volatile, become incandescent at the same temperature. Dr. Draper concludes from his experiments that, from common temperatures up to 977°, the rays emitted by a heated solid, are invisible (black heat). At this temperature they are red, and as the heat of the body increases continuously, other rays are added, increasing in refrangibility as the temperature rises.

A mass of gold, heated in glass or porcelain, will acquire a red heat at the same time as the glass or porcelain in which it is heated. Lead melted in a ladle may be made red-hot by continuing the heat above fusion. The red rays are emitted by the melted lead, and are plainly visible in the dark. The iron ladle, in which the metal is liquefied, will also appear red-hot at the same time. Above this temperature a solid body when heated, becomes orange-yellow, and ultimately white.

According to Bunsen, between a yellow-red, and white heat, the colours of intensely heated bodies pass through shades of blue to violet, while the white heat is the resultant of all the spectral colours emitted by the heated substance.

From what is said in the last and in preceding paragraphs, it is evident that the thermometer gives us very limited information with respect to heat: it merely indicates, in fact, what may be called the tension of heat in bodies, or the tendency of the heat to spread from them. Thus it does not enable us to discover that a pound of water takes thirty times as much heat to raise its temperature one degree, as a pound of mercury; nor does it indicate the heat of fluidity when bodies change their form.

"Heat by its different relation to different substances has a

powerful influence on their chemical combinations."

720. By observations made and recorded through past ages, man

any accuracy. Hence this may be regarded as the lowest boundary of the thermometer which can be practically reached. All other liquids are solidified long before reaching this temperature.

508

Chemical Effects of Heat.

has now come to know that the substances constituting the world around him, although appearing to differ in their nature almost to infinity, are yet all made up of a few simple elements variously combined; and he has discovered that the peculiar relations of these elements to heat, particularly their being unequally expanded by it, and their undergoing fusion and vaporization at different temperatures, furnish him with ready means of separating, combining, and new-modifying them to serve to him purposes of the highest utility. In many instances no chemical combination can take place without the application of heat, and it may be regarded in all cases as a necessary agent to the chemist. It either causes substances to combine or it separates those which are already combined. Many examples of this duplex operation have been already given in the preceding pages.

A piece of cold charcoal may lie in the air for centuries without change, but when heated to about 1000° it combines with one of the elements of air-oxygen, and produces the phenomenon of combustion, being at the same time entirely converted into an invisible gas (carbonic acid). In this case, according to modern views, the chemical force here represents motion, and the motion of chemical force is supposed to be converted into the motion of heat. Thus heat is evolved during combination, because the chemical force that gives rise to the combination, is always more or less converted into heat. In some cases a portion of the chemical force is converted into electricity, and in others into light.

To take another example. Sulphur and iron have no tendency to unite at ordinary temperatures; but if a bar of iron is made fully red hot and a stick of sulphur is applied to it, the two elements quickly disappear and melt into a new compound, sulphide of iron. A thick sheet of red-hot iron may thus be pierced by pressing against it a roll of sulphur. Sulphur manifests no tendency to combine with indiarubber in the cold; but if a sheet of this substance is dipped into melted sulphur at about 300°, a combination takes place, and a substance well known as vulcanized rubber is produced. The rubber retains from ten to sixteen per cent. of sulphur, and its properties are entirely changed. It is not affected by heat or cold; it is rendered much more elastic, and at the same time insoluble in all liquids which dissolve rubber. It does not soften and melt by heat like either of its two constituents, and it thus acquires a number of properties which render it far more serviceable in the arts and manufactures than the natural rubber. The heat required

Chemical Effects of Heat.

509

for vulcanization is remarkably well defined; if too low, there is no combination; if too high, a hard, dark compound like ebony is obtained..

721. There are some substances which do not require to be heated in order to effect combination. Mere contact at the lowest temperature is sufficient. Thus, when iodine is placed on a slice of phosphorus, both at a temperature of 32°, there is immediate combination of the two elements, with the production of a white heat and a most intense light. So when sodium wrapped in blotting paper is placed in a hole on a block of ice, it suddenly bursts into flame and produces a brilliant combustion. The illustrations, which might be produced, of the effect of heat in evolving chemical force or in producing chemical changes, are almost endless. In fact, it may be doubted whether any such changes can take place without the intervention of heat in some form. When dry iodide of nitrogen is allowed to fall through the air, it explodes. In this case the heat arising from the slight friction by contact with the particles of air is sufficient to separate the elements and produce an explosion like that caused by direct pressure or percussion. Powdered chlorate of potash, mixed with allotropic phosphorus, explodes with violence when mixed with a feather or a slip of paper. It is the heat arising from friction, however slight, which operates in these cases.

Heat can make and unmake chemical compounds of the same elements. The elements of water-hydrogen and oxygen-may remain mixed for centuries without combining at ordinary temperatures; but when a red heat is applied to the mixture, they instantly combine with explosion, and form liquid water. When a white-hot ball of platinum is introduced into this liquid water and a tube for collecting gases is placed over it, the oxygen and hydrogen are devolved, and may be collected as independent gases mixed in the proportions to form water. The heat required for their combination is 1000°, and for their separation at least 3000°.

722. The most wonderful effects of heat on the chemical and physical properties of bodies are, however, met with in some forms of allotropy.* Phosphorus, sulphur, and other substances are so altered by heat that they are no longer recognizable as the same elements. Common phosphorus is a white, waxy-looking substance, —combines with oxygen at all temperatures, and is luminous in the dark-highly inflammable at a low temperature (113°), soluble in *From the Greek, Aλos, other or different, and Tρéπew, to turn-sig nifying change of state.