3J2 ACTION OF HEAT MOBlFIEt* 



Explanation of stances, their hardness, compact appearance, susceptibility of 



the table of y^^^ ^^^ ^^-^^ j^ ^^^ ^^^^j^^ 



specthicgravi- * ° 



ties, &c. The case is widely different, when we attend to the results of 



the common method contained in column TX. Here the spe- 

 cific gravity of chalk is rated at 2.498, which exceeds consider- 

 ably that of a majority of the result tried. Thus, it would ap- 

 pear, by this method, that chalk has become lighter by the 

 experiment, in defiance of our senses, which evince an increase 

 of density. 



This singular result arises, I conceive, from this, that, in our 

 specimens, the faculty of absorption has been much moi'e de- 

 creased than the porosity. Thus, if a piece of crude chalk, 

 whose specific gravity had previously been ascertained by the 

 common mode, and then well dried in a heat of 212°, were 

 dipped in varnish, which would penetrate a little way into its 

 surface ; and, the varnish having hardened, the chalk were 

 weighed in water, it is evident that the apparent loss of weight 

 would now be greater by 23.6l per cent, of the dry weight, than 

 it had been when the unvarnished chalk was weighed in water ; 

 because the varnish, closing the superficial pores, would quite 

 prevent the absorption, while it added but little to the weight 

 of the mass, and made no change on the bulk. In computing, 

 then, the specific gravity, by means of this last result, the chalk 

 would appear very much lighter than at first, though its density 

 bad, in fact, been increased by means of the varnish. 



A similar effect seems to have been produced in some of 

 these results by the agglutination or partial fusion of part of the 

 substance, by which some of the pores have been shut out from 

 the water. 



This view derives some confirmation from an inspection of 

 columns VI. and VII.; the first of which expresses the ab- 

 sorption; and the second, that result, reduced to -a per centage 

 of the original weight. It there appears, that whereas chalk 

 absorbs 23.97 per cent., some of our results absorb only 0.5, 

 or so low as 0.1 1 per ccjit. So that the power of absoption has 

 been reduced from about one-fourth, to less than the five hun- 

 dredth of the weight. 



I have measured the diminution of bulk in many cases, par- 

 ticularly in that of No. 1 1. The chalk, when crude, ran to 

 th. .':,th degree of Wedgwood's gage, and shrunk so much du- 

 ring the expcrmieut, tbAt it ran to the ItJlst; the difference 



amount- 



