Effects of Heat modified by Compression. 2 1 3 



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 

 jnto 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*61 per cent, of the 

 dry weight than it had been when the unvarnished chalk was 

 weighed in water; because the varnish, closing the super- 

 ficial 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 gra- 

 vity by means of this last result, the chalk would appear very 

 much lighter than at first, though its density had, 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 

 absorption; 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*11 percent. So that the 

 power of absorption has been reduced from about one-fourth 

 to less than the five hundredth of the weight. 



I have measured the diminution of bulk in many cases, 

 particularly in that of No. n. The chalk, when crude, 

 ran to the 73th degree of Wedgcwood's gage, and shrunk so 

 much during the experiment, that it ran to the 1 6lst. ; the 

 difference amounting to S6 degrees. Now, I find that Wedge- 

 wood's gage tapers in breadth, from 0-5 at zero of the scale, 

 to - 3 at the 240th degree. Hence we have for one decree 

 O-00OS33. Con-uqucntly, the width, at the 75th degree, 

 amounts to 0'AZ"ibQ.b\ and at the ldlst, to 0-3658S7. These 

 numbers, denoting the linear measure of the crude chalk, 

 and of its refill under heat and compression, arc as JOO to 

 63-8; or, in solid bulk, as 100 to 57"3. Computing the 

 densities from Shis source, they are as ] to 1*73. The spe- 

 O 3 cific 



