326 PROCEEDINGS OF THE ACADEMY OF [1880. 



to a white heat without exfoliation, and when afterwards held in 

 the flame, exfoliate but slightly. Apparently the water in Phila- 

 delphite is combined precisely as in the zeolites 



It appears that it is as difficult to make a distinction between 

 water of crystallization ahd water of constitution as it is to make 

 one between the former and li3^groscopic water. 



Amount of Exfoliation. — The amount of exfoliation is quite 

 constant at ten times the original volume. 



Original thickness. Thickness after exfoliation. 



Inches. Inches. Eatio, 



.015 .13 



.015 .17 



.02 .2 1 : 10 



.03 .25 



mean 1 : 10 



04 .47 I mean 1 : 10.4 



.06 .68 



/ 



These experiments were made by heating tlie mica on platinum 

 foil over the flame of a Bunsen burner. The heat must be sudden 

 in order to have a large exfoliation (i-. Exp't. k\ Exfoliation 

 takes place in one direction onlj', viz., at right angles to the 

 cleavage. No lateral expansion whatever occurs. When the 

 flame is applied to one side of the mica, that side exfoli^es the 

 most, and causes the exfoliating mineral to curve in the opposite 

 direction. 



Force of Exfoliation. — It has been found that the force exer- 

 cised during the exfoliation of Philadelphite is enormous. In one 

 experiment a fragment of it while exfoliating lifted more than 

 50,000 times its own weight. The 'force of exfoliation is governed 

 by a law which is the inverse of that controlling the exfoliation 

 temperature. It may be stated thus : The force of exfoliation 

 increases directly with the rapidit}' of the expulsion of water, and 

 inversel}^ with the volume of the substance. The latter part ot 

 the law follows as a necessaiy consequence of the first part, since 

 the smaller the fragment, the more rapidl}- and completel}' can it 

 be heated. Various experiments were made, and though per- 

 lormed in an extremeh^ rough manner, will give an idea of this 

 force. To find what amount a given Aveight of the mica could lift 

 when exfoliating, iron pound-weights were placed upon the ring of 

 a retort stand and connected with the fragment of mica placed on 

 a support immediately below them. A pencil of chalk or gas 



