412 On a new Mode of Artificial Congelation. 



tained; yet, though sound philosophy tends always to simplifi- 

 cation, the rare quality of simplicity is scarcely ever the flash of 

 intuition, but the slow fruit of close and patient investigation. 



In pursuing the researches with his hygrometer, Professor 

 Leslie was early induced to inquire into the condition of the 

 higher atmosphere, and its relations to humidity. He thus de- 

 tected a fact of great importance in melearology, and pointing 

 at various ulterior views. 



Investigation of its Principles. 



As rarefaction enlarges the capacity of air for heat, so it like- 

 wise augments the disposition to hold moisture ; at the same 

 time, that the removal of the ordinary pressure facilitates the ex- 

 pansion of the liquid matter, and its conversion into a gaseous 

 form. Accordingly, if the hygrometer he suspended within a 

 large receiver, from which a certain portion of air is quickly alj-- 

 tracted, it will sink with rapidity. In summer the additional 

 dryness thus produced amounts to about 50 hygrometric degrees, 

 each time the air has its rarefaction doubled ; so that, sup- 

 posing the operation of exhausting to be performed with expe- 

 dition, and the residuum reduced to a sixty-fourth part, the iiv- 

 grometer would mark a descent of 300°. But this effect is only 

 momentary; for the thin air very soon becomes charged with 

 moisture, and, consequently, ceases to act on the wet ball of the 

 hygrometer. The cold, however, excited on the surface of that 

 ball, by such intense evaporation, will have previously frozen the 

 coating. 



The increased power of aqueous solution which air acquires as 

 it grows thinner, being ascertained and carefully investigated, 

 the object was to combine the action of absorbent with the tran- 

 sient dryness produced within a receiver by rarefaction. The 

 sentient ball of the hygrometer being covered with dry salt of 

 tartar, the instrument first indicated increasing dryness, and 

 afterwards, as the rarefaction proceeded, it changed its course, 

 and marked humidity. The same variation of. effect nearly was 

 observed, when the hygrometer had been wetted as usual with pure 

 water, and a broad saucer containing the n)ild vegetable alkali 

 was placed on the plate of the air-pump. It was thus proved, 

 that the action of this imperfect absorbent is soon overjiowered 

 by the tendency to vaporization in attenuated air, and that, be- 

 yond a certain limit, it surrenders its latent moisture. 



Mr. Leslie resolved, therefore, to try the effect of sulphuric 

 acid, whose peculiar energy as an absorbent he had, under other 

 circumstances, already ascertained. But various incidents pre- 

 vented him, for a considerable time, from resuming his philoso- 

 phical inquiries. At labt he began those projected expcrimentSj 



and 



