ON A NEW VARIETY OF HALLOYSITE FROM SERVIA. 213 



of the same density throughout. By geometrical reason- 

 ing he arrives at the conclusion that the curve of intensity 

 (the gradulucique, as he terms it) has this property : its 

 suhtangent multiplied by the density is equal to a con- 

 stant. Expressed in the language of the differential 

 calculus, this gives rise to a differential equation similar 

 to the one which I obtained by a different method, and 

 gave last session in a paper read before the Society on the 

 intensity of light which has traversed a medium wherein 

 the density is some function of the distance traversed. 

 Except in the consideration of the intensity of light which 

 has passed through the atmosphere, Bouguer has made 

 very little use of this highly general theorem ; for, says he, 

 in most cases we do not know what is the law of density. 

 This may be so, but by assuming the density to be some 

 function of the distance, we may deduce some interesting 

 and valuable results. 



XX. On a New Variety of Halloysite from Maidenpeh, 

 Servia. By H. E. Roscoe, LL.D., F.R.S., Presi- 

 dent. 



Read January 22nd, 1884. 



This mineral, one of the few peculiar to Servia, was 

 given to me by Mr. James Taylor, lately a resident at 

 Maidenpek. It is a very soft (h=2*5) whitish green non- 

 crystalline mineral, having a conchoidal waxy fracture. 

 It is translucent in thin films, but opaque in mass, and 

 adherent to the tongue. Its specific gravity is 2*07. On 

 exposure to air it loses a portion of its combined water, 



