222 



WRITINGS OF JOSEPH HENRY. 



[1855- 



ration as well as to call forth our gratitude on account of 

 the important truths which it reveals. Dalton, although 

 a profound thinker, and thoroughly imbued with a love of 

 science for its own sake, was eminently a practical man in 

 the proper sense of the term. He had not only the sagacity 

 to frame significant questions to be propounded to Nature, 

 but also the ingenuity to devise simple means by which the 

 answers to these questions would be given in terms the most 

 precise and accurate. 



The weight of vapor. — There are other important questions 

 to be answered in regard to the same subject; and the first 

 we shall consider is the relative weight of a given quantity 

 of vapor in a space fully saturated at different temperatures. 

 The general method of ascertaining the weight of a given 

 quantity of an aeriform fluid consists in weighing a vessel 

 of known capacity when exhausted, and again when it is 

 filled with the air or vapor of which the weight, or in other 

 words the density, is desired. The difference of weights of 

 the vessel in the two conditions evidently gives 

 the weight required. This may serve to give a 

 general idea of the method of determining the 

 weight of vapor; but it may be well to dwell a few 

 moments on a more detailed account of one of the 

 processes which has been actuall}^ adopted. This 

 consists in employing an apparatus formed of a 

 glass globe a (Fig. 3) screwed at / to the top of 

 a barometer tube e. The capacity of the globe is 

 previously ascertained by weighing it empty and 

 afterwards filled with mercury. The difference of 

 weight gives the weight of mercury sufficient to 

 fill it, and from this it is easy to calculate its con- 

 tents in cubic inches or parts of a cubic foot. Next, 

 a small hollow bulb of glass g, is formed by the 

 blow-pipe, and filled with a known weight of water. 

 For this purpose the capillary tube c (Fig. 4, in 

 which the bulb ^f is represented much enlarged) 

 is plunged beneath a surface of water 6, and the 

 glass gradually heated by a spirit lamp d, by which the air 



Fig. 3. 



