CH. XXVI.

on now,

WILLIAM SMITH.

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and that the only way to read the history of the past is to compare it with the present.

William Smith surveys the Rocks of England.—Meanwhile another man, whom we must not forget to mention, was working away very quietly without any help, and with very little money; and yet in his way was doing at least as much work as the others. This was William Smith, a plain English surveyor, who was so much struck with the arrangement of the different formations in the hills among which he travelled that he determined to try and map them out so as to show exactly how the strata are placed one above the other, and what counties they pass through.

He began his work in 1790, and travelled over the whole country, chiefly on foot, marking as he went all the different positions of the rocks, and collecting the shells and other fossils which he found in them. He had not gone on long before he observed that certain fossils which appeared in the lower beds disappeared when he reached those which lay above them, and that others took their place; so that in this way it was possible to use the fossils to trace out the age of any particular rock, just as the face of a coin helps you to tell the reign in which it was cast; and the story told by the fossils agreed very well with the divisions which he had worked out by the position of the rocks above each other. He was even so observant that he distinguished between the fossils which had their edges fresh, showing that they had not been disturbed since they were buried in the earth, and those which were rubbed and water-worn. The fresh ones only, he said, are of use to tell the age of a rock, for those which are rubbed may have been washed out of some older formation by rivers.

In this way William Smith, for pure love of science, and

without any hope of gain, travelled over the whole of England and Wales, mapping out the rocks and noticing all their peculiarities. In 1799 he published a list or tabular view of the formations with their fossils, and the places where they might be seen in the hills; and in 1815 he at last succeeded in completing a geological map of England, which has ever since formed the foundation of our British geology, and which remains a lasting monument of what one man may accomplish by patience and indefatigable industry. William Smith fairly earned the title of the 'Father of English Geologists,' which has ever since been given him, and, with Werner and Hutton, deserves to be remembered as one of the founders of the science of geology.

Chief Works consulted.—Lyell's 'Principles of Geology;' Lyell's 'Student's Elements of Geology;' Page's Advanced Text-Book of Geology;' Hutton's Theory of the Earth;' Fitton's Notes on Progress of Geology in England;' 'Life of Werner' — 'Naturalists' Library,' vol. xxxix.

CH. XXVII.

MODERN CHEMISTRY.

225

CHAPTER XXVII.

SCIENCE OF THE EIGHTEENTH CENTURY (CONTINUED). Birth of Modern Chemistry-Discovery of 'Fixed Air,' or Carbonic Acid, by Black and Bergmann - Working out of 'Chemical Affinity' by Bergmann-He tests Mineral Waters, and proves 'Fixed Air' to be an Acid-Discovery of Hydrogen by Cavendish -He Investigates the Composition of Water-Oxygen discovered by Priestley and Scheele—Priestley's Experiments—He fails to see the true bearing of his Discovery-His Political Troubles and Death -Nitrogen described by Dr. Rutherford-Lavoisier lays the Foundation of Modern Chemistry-He destroys the Theory of 'Phlogiston' by proving that Combustion and Respiration take up a Gas out of the Air-Discovers the Composition of Carbonic Acid and the nature of the Diamond-French School of ChemistryDeath of Lavoisier.

DURING the last half of the eighteenth century, while Hunter and Linnæus were adding to our knowledge of living beings, and Werner and Hutton were reading the history of the crust of the earth, a little group of men in England, France, and Sweden were making discoveries which entirely altered the science of chemistry. These men were Bergmann and Scheele in Sweden; Black, Cavendish, and Priestley in England; and Lavoisier in France.

In order to understand what their discoveries were, and what they taught us, it is necessary to bear in mind that up to this time chemists had believed fire, air, and water to be simple substances which could not be decomposed or split up into any other kind of matter. Mayow, indeed, had

shown that the atmosphere could be separated into two gases, but his experiments had been passed over and forgotten; and though Dr. Hales, at the beginning of the eighteenth century, had collected several gases, he had not distinguished them from air. The fact was that Stahl's imaginary 'phlogiston,' which was supposed to pass out of burning and breathing bodies into the air, was a constant source of confusion, and led men away from the truth.

But the time had now come when these misty ideas were to be dispelled, by the discovery of the four gases-carbonic acid, hydrogen, oxygen, and nitrogen.

Discovery of 'Fixed Air,' or Carbonic Acid, by Black, 1756. The first step was made by a Scotch physician named Black, who was born in 1728, and became Professor of Chemistry at Glasgow in 1756. Here he made many valuable experiments, and among other things he was very anxious to find out why limestone altogether changes its character when it is burnt. If you take a piece of ordinary limestone or chalk, and put it in water, it will remain without any change unless you add a little acid to the water, and then the limestone will effervesce, and bubbles will begin to rise up from it. But if you take a piece of the same limestone and burn it in a fire, it turns into a powder called quick-lime, which will no longer give out bubbles when you pour acid upon it, but directly you mix it with water it will swell up and become intensely hot, as you may see for yourself if you watch bricklayers making mortar by the roadside. This complete change in the limestone, caused by merely heating it, had been a great problem to chemists; and Dr. Black was still more puzzled by finding that the lime was lighter after it had been burnt, although he could not discover that it had lost anything except a little

CH. XXVII.

BLACK'S FIXED AIR."

221

water, which was not enough to account for the loss of weight.

At last he remembered that Dr. Hales had driven air out of substances, and collected it in bottles; and he began to consider whether the heat of burning might not have driven some heavy kind of air out of the limestone, and so made it Lighter. To prove this he made the experiment which has since been always used for making small quantities of carbonic acid gas. He put some pieces of limestone in the bottle, a, Fig. 38, and poured upon them some water and

Carbonic Acid rising from Limestone and Acidulated Water (Griffin). Bottle containing pieces of limestone in water and acid. b, Connecting tube, c, Inverted jar, out of which the rising gas is driving the water.

some acid. He then stopped the bottle with a tight cork, and joined it by the tube b to a large glass jar, c, filled with water, and standing with its open end downwards in a vessel of water. In a few moments the bubbles began to rise from the limestone, and passing into the jar, c, drove out the water and filled the jar with gas.

This gas Black called 'fixed air,' because it had been fixed in the limestone before it was driven out by the acid. He collected and weighed it, and found that it exactly