More Original Essays.

139

This law of

whether the space contains air or not.' Dalton was verified by Gay Lussac, but recently Regnault has found, by more exact experiments, that the pressure in air is always less (about 2 per cent.) than that in vacuo; he is, however, inclined to believe that Dalton's law is true in principle, and that the deviations which he noticed are to be explained by the hygroscopic character of the walls of the chamber which contained the vapour.'

His essay "On the Constitution of Mixed Gases, and particularly of the Atmosphere," was not favourably received, inasmuch as his views were not so very clearly expressed, and afterwards received much modification at his own hands. The history of the controversy will be found in his "New System of Chemical Philosophy," Part i. pp. 150-193.

His second essay "On the Force of Steam or Vapour from Water, and various other Liquids, both in a Vacuum and in Air," was deemed by Dr Henry as one of transcendent importance, as first furnishing tabulated (data for the solution of perhaps the most interesting problem in meteorology; namely, the calculation, after noting the dew point, of the absolute quantity of moisture in a given volume of air. The first sentence of the essay contains the anticipation of a discovery subsequently made by Dr Michael Faraday. Dalton's words are:-"There can scarcely be a doubt entertained respecting the reducibility of all elastic fluids, of whatever kind, into liquids; and we ought not to despair of effecting it in low temperatures, and by strong pressure exerted upon the unmixed gases."

His experiments made on the vapours of sulphuric

ether, spirits of wine, water of ammonia, solution of muriate of lime, mercury and sulphuric acid, led him to entertain as a general law, "that the variation of the force of vapour from all liquids is the same for the same variation of temperature, reckoning from vapour of any given force; thus, assuming a force equal to 30 inches of mercury as the standard, it being the force of vapour from any liquid boiling in the open air, we find aqueous vapour loses half its force by a diminution of 30° of temperature; so does the vapour of any other liquid lose half its force by diminishing its temperature 30° below that in which it boils, and the like for any other increment or decrement of heat." Nothing could well appear more unlike in character than the six liquids operated upon by Dalton; and though the law he laid down does not universally obtain, "it is nevertheless remarkable," observes Gmelin, "that this law is pretty nearly true in the case of many substances." Dalton's views received further sanction at the hands of such great authorities as Arago, Faraday, and Dove, who have shown that although the hypothesis does not hold generally, it is approximately true for short distances on each side of the boiling point in a large number of instances.

Though Dalton experimented largely before offering his opinions "On Evaporation," it does not seem needful to do more than draw attention to the fact of its appearing with his other essays of greater import. His essay, "On the Expansion of Elastic Fluids by Heat," arising, in part, from a discussion in which several French savans took part, led to greater results. Dalton ascertained by repeated experiments that 1000 volumes of common air of the temperature 55°

The Expansion of Elastic Fluids.

141

and common pressure expand to 1325 volumes, when heated to the temperature of 212°, and he concluded that any gas at any temperature increases in volume for a rise of one degree by a constant fraction of its bulk at that temperature. He also found that hydrogen, oxygen, carbonic acid gas, and nitrous gas, expand to the same amount as common air; the minute differences observed being attributable to the presence of aqueous vapour. Gay Lussac obtained in the same year (1801) results differing but slightly from those of Dalton, the expansion for a single degree of Fahrenheit being, according to Gay Lussac,

1

of the primitive volume at 32°, and according to Dalton Magnus and Regnault, by more exact experiments, have determined the expansion to be

, and their experiments leave little doubt that Gay Lussac's method of expressing the law is much nearer the truth than Dalton's.

From these experiments Dalton was led to conclude "that all elastic fluids, under the same pressure, expand equally by heat, and that for any given expansion of mercury the corresponding expansion of air is proportionally something less, the higher the temperature. It seems, therefore, that general laws respecting the absolute quantity and the nature of heat are more likely to be derived from elastic fluids than from other substances. As every other liquid we are acquainted with is found to expand more in the higher than in the lower temperatures, analogy is in favour of the conclusions of De Luc, that mercury does the same." It is scarcely possible, writes Dr Henry, p. 37-8, to over-estimate the value of these sagacious conclusions. They may be

affirmed to lie at the basis of the profound and hitherto unrivalled Memoir, by MM. Dulong and Petit, on the "Measure of Temperature.” . . . "It is well known that their singularly precise experiments signally confirmed Dalton's sagacious inferences from his less exact researches."

Dr Henry has dwelt at some length on these four remarkable essays, because, as he says, "independently of their momentous bearing on meteorological science, they are deeply stamped with the impress of Dalton's genius, and furnish instructive types of his modes of working and thinking. His instruments of research, chiefly made by his own hands, were incapable of affording accurate results, and his manner of experimenting was loose, if not slovenly. His numerical determinations have not, therefore, like even the earlier analyses of Prout, been confirmed by subsequent inquiries. Still his experiments, though wanting the exactitude of modern research, were not unskilfully devised, and were most sagaciously interpreted. They were, perhaps, such as were most needed at the close of the last century, when so many fields of experimental research were untilled, that bold tentative incursions into new domains of thought, large groupings, and happy generalisations of approximate results were more effective instruments of advance than scrupulous precision in details. At all events, from these imperfect experiments, Dalton arrived at the discovery of those general laws of evaporation, and of the relation of air with moisture, which were translated by Biot into the exact language of analytical formulæ, and which still constitute the foundation of meteorological science."

CHAPTER VIII.

"There are more things in heaven and earth, Horatio,
Than are dreamt of in your philosophy."—

SHAKESPEARE.

ELEMENTS OF ENGLISH GRAMMAR-AN

EXCURSION-VARIETY OF

CORRESPONDENCE-GRAMMAR AND PUPILS-THE ATMOSPHERE
-FIRST INDICATIONS OF MULTIPLE PROPORTION - - ELASTIC
FLUIDS-ABSORPTION OF GASES-ATOMIC WEIGHTS AND INDEX

TO ATOMIC THEORY.

ALTON'S educational and scientific walk was varied and laborious to a degree almost unprecedented in the pages of biography. His faithful plodding truly merited, and in due time, met with paramount success. Happily, in his struggles for a bread-and-butter existence, and his more earnest solicitation for philosophical inspiration, he possessed a hardy and robust constitution, and a northern temperament that found its most fitting stimulus in mental efforts and continuous work. How few men, even of the hardy Teutonic race, could have sustained the long and tedious hours that saw Dalton engrossed with the duties of teaching arithmetic and grammar to "young ideas!" Yet these compulsory engagements were as the playthings to his mind, whose proper pabulum was to be found in experimental research and abstruse inquiries.