410 Mr. W. Stokes on [June, 



are equal to 48 - 68, agreeing with the above result as nearly as 

 could be expected. This proportion of water was confirmed by 

 mixing 1 1*16 parts of the alum with a solution of 13*06 of nitrate 

 of barytes, which are the proportions required by calculation on 

 the supposition that the salt contains 24 atoms of water ; there 

 was a slight excess of sulphuric acid, but the addition of nitrate 

 of barytes did not occasion any turbid ness for. some minutes. 



This alum is, therefore, composed of three atoms of sulphate of 

 alumina, one atom of sulphate of ammonia, and 24 atoms of 

 water; and the triple salt now described consists of 3 atoms 

 of persulphate of iron, 1 atom of sulphate of ammonia, and 

 24 atoms of water ; or if we adopt the number assigned by 

 Dr. Thomson to peroxide of iron, 1 atom of tripersulphate of 

 iron, 1 of sulphate of ammonia, and 24 atoms of water. 



Article III. 



On Clay Slate for Road-making. By Mr. W. Stokes. 



(To the Editor of the Annals of Philosophy.) 



SIR, Ilarcourt-strcct, Duhlitt. 



In many parts of Ireland is found a species of clay slate which 

 answers remarkably well for road-making. This clay slate may 

 be described as follows : 



It occurs massive; colour of the fresh fracture, a dark bluish- 

 grey. The colour of that which has been exposed to the air is 

 a brownish-red. Lustre, in some places, greasy ; in some exhi- 

 biting brilliant points : a few places have a blistered appearance. 

 The fracture undulating, slaty ; yields readily to the knife ; it 

 feels slightly greasy ; opaque ; specific gravity, 2*6. Before the 

 blowpipe, it melts into a greenish slag. 



The advantages which this stone possesses as a road material 

 are remarkable, and might seem inconsistent were they not veri- 

 fied by experience. 



Although easily broken it does not form light dust so as to 

 rise with the wind ; it is not readily worked into mud by car- 

 riage wheels or the feet of cattle, although roads made of other 

 species of clay slate are very liable to this defect. Roads formed 

 of this peculiar clay slate have an agreeable smoothness under 

 the horse's feet, and seldom exhibit projecting stones ; neither 

 do they throw out loose stones, which is a common defect of 

 our limestone roads. They preserve their evenness in decay 

 remarkably, continuing smooth when only an inch of gravel re- 

 mains on the surface ; they are 30 porous that even though they 

 have lost their central elevation, water does not rest on them. 



