Heroes of Science: Chemists

by M. M. Pattison Muir

On one occasion during Dalton's stay at Kendal, as he was about to make a visit to his native village, he bethought himself that the present of a pair of silken hose would be acceptable to his mother. He accordingly purchased a pair marked "newest fashion;" but his mother's remark, "Thou hast brought me a pair of grand hose, John; but what made thee fancy so light a colour? I can never show myself at meeting in them," rather disconcerted him, as to his eyes the hose were of the orthodox drab colour. His mother insisted that the stockings were "as red as a cherry." John's brother upheld the "drab" side of the dispute; so the neighbours were called in, and gave their decision that the hose were "varra fine stuff, but uncommon scarlety."

From this time Dalton made observations on the peculiarities of his own vision and that of others, and in his first paper read before the Literary and Philosophical Society in 1794, he described these peculiarities. He says, "Since the year 1790 the occasional study of botany obliged me to attend more to colour than before. With respect to colours that were white, yellow, or green, I readily assented to the appropriate term; blue, purple, pink and crimson appeared rather less distinguishable, being, according to my idea, all referable to blue. I have often seriously asked a person whether a flower was blue or pink, but was generally considered to be in jest." Dalton's colour-blindness was amusingly illustrated at a later time, when having been created D.C.L. by the University of Oxford he continued to wear the red robes of his degree for some days; and when his attention was drawn to the somewhat strange phenomenon, even in a university town, of an elderly gentleman in the dress of a Quaker perambulating the town day after day in a scarlet robe, he remarked that to him the gown appeared to be of the same colour as the green trees.

Dalton's work during the next six or eight years dealt chiefly with problems suggested by his meteorological observations; he published a volume on "Meteorological Observations and Essays," chiefly occupied with descriptions of the instruments employed, more especially of the thermometer and barometer, and an instrument for determining the dew-point of air. By this time he had established the existence of a connection of some kind between magnetism and the aurora, and had thus laid the foundations of a most important branch of meteorology.

In 1799, in a note to a paper on rain and dew, he begins his work on aqueous vapour in the atmosphere by proving that water vapour exists as such in the air. This paper is quickly followed by another on the conducting power of water for heat.

A very important paper was published in 1801, on the "Constitution of Mixed Gases, etc.," wherein Dalton asserted that the total pressure of a mixture of two gases on the walls of the containing vessel is equal to the sum of the pressures of each gas; in other words, that if one gas is removed the pressure now exerted by the remaining gas is exactly the same as was exerted by that gas in the original mixture. In a paper published much later (1826), when his views and experiments on this subject were matured, he writes: "It appears to me as completely demonstrated as any physical principle, that whenever two or more ... gases or vapours ... are put together, either into a limited or unlimited space, they will finally be arranged each as if it occupied the whole space, and the others were not present; the nature of the fluids and gravitation being the only efficacious agents."

This conclusion was followed out and extended in a paper published in 1803, on the absorption of gases by water and other liquids, wherein he states that the amount of each gas mechanically dissolved by a liquid from a mixture of gases depends only on the quantity of that gas in the mixture, the other gases exerting no influence in this respect.

Dalton now considered the variation in the pressures of various gases caused by increasing or decreasing temperature, and then proceeded to discuss the relations which exist between the volumes of gases and the temperature at which these volumes are measured. He concluded that "all elastic fluids" under the same pressure expand equally by heat: and he adds the very important remark, "It seems, therefore, that general laws respecting the absolute quantity and the nature of heat are more likely to be derived from the study of elastic fluids than of other substances"—a remark the profound truth of which has been emphasized by each step in the advances made in our conception of the nature of heat since the time of Dalton.

In these papers on the "Constitution of Mixed Gases" Dalton also describes and illustrates a method whereby the actual amount of water vapour in a given bulk of atmospheric air may be found from a knowledge of the dew-point of that air, that is, the temperature at which the deposition of water in the liquid form begins. The introduction of this method for finding the humidity of air marks an important advance in the history of meteorology.

In this series of papers published within the first three years of the present century Dalton evidently had before his mind's eye a picture of a gas as a quantity of matter built up of small but independent particles; he constantly speaks of pressures between the small particles of elastic fluids, of these particles as repelling each other, etc. In his "New System" he says, "A vessel full of any pure elastic fluid presents to the imagination a picture like one full of small shot."

It is very important to notice that Dalton makes use of this conception of small particles to explain purely physical experiments and operations. Although we know that during these years he was thinking much of "chemical combinations," yet we find that it was his observations on the weather which led him to the conception—a purely physical conception—of each chemically distinct gas as being built up of a vast number of small, equally heavy particles. A consideration of these papers by Dalton on the constitution of mixed gases shows us the method which he pursued in his investigations. "The progress of philosophical knowledge," he says, "is advanced by the discovery of new and important facts; but much more when these facts lead to the establishment of general laws." Dalton always strove to attain to general laws. The facts which he describes are frequently inaccurate; he was singularly deficient in manipulation, and he cannot claim a high place as a careful experimenter. He was however able to draw general conclusions of wide applicability. He seems sometimes to have stated a generalization in definite form before he had obtained any experimental verification of it.