176 Lero'i and Saussures Method of taking the Dew-Point. 



that had imbibed it ; whereas, where nitrate of strontian was 

 employed, the functions of life appeared to go on for a consider- 

 able time without material obstruction. 



Upon the whole, then, I see nothing, so far as experiments 

 have yet gone, to invalidate the conclusion, to which the preced- 

 ing facts appear to lead, that the roots of plants do, to a certain 

 extent at least, possess a power of selection, and that ihe earthy 

 constituents which form the basis of their solid parts are deter- 

 mined as to quality by some primary law of nature, although 

 their amount may depend upon the more or less abundant sup- 

 ply of the principles presented to them from without. 



Note on Taking the Dew-Point directly, by the Method of 

 Leroi and Saussure. 



The following observations aim at nothing new in point of 

 principle, and are merely intended to facilitate the long known 

 and simple mode of finding the dew-point, by the cooling action 

 of saline solutions on polished surfaces. The arrangement is by 

 no means suggested as entering into competition with more cost- 

 ly and ingenious hygrometric instruments ; but will probably be 

 found useful in the absence of such aids. 



The objects proposed are, first, to reduce within very narrow 

 limits the quantity of materials, and size of the vessel used, and 

 thereby extend the circumstances under which this method may 

 be adopted ; and, secondly, to avoid the trouble of any trans- 

 ference of the materials from one vessel to another, or of any 

 successive additions of the substances in the course of the same 

 experiment, by producing the reduction of temperature simply 

 by a gentle and carefully regulated agitation of the vessel. 



A little hollow ball, or bottle of thin brass, like that in the op- 

 posite page, is employed, having a diameter of 1 1 inch. The necTc 

 is ^-^ to ^"g of an inch long, and about -^^ wide, and is farther 

 expanded into a funnel shape at top. The bottle, when filled to 

 the top of the neck, contains about \ an ounce. The outside is 

 preserved bright and polished ; and it will save trouble in clean- 

 ing, although not answer the purpose better, to have the out- 

 hide gilt. A small thermometer is used, having a very fine 



