262 Mr Sang's Annual Report on the 



approach to a simple proportion ; here it almost appears as if 

 the quantity of oxygen of the silica were three times as great as 

 that of the bases, and that the clay-slate contained neutral com- 

 binations of silica ; although the proportion of the silica is 

 throughout too large, and the diiferences too great, to be ascribed 

 to errors in the analysis. It is therefore apparent, that the 

 clay- slate of the transition series is not a simple mineral, an opi- 

 nion proved beyond a doubt by the result of treatment with 

 acids ; but the fact of the composition of the mineral ingredients 

 into which clay-slate can be separated, not corresponding with 

 the doctrine of fixed proportions, proves that the clay-slate is not 

 to be regarded as a compound of two simple minerals, but as 

 the product of the decomposition of other mountain rocks ; but 

 the close agreement in composition, of the varieties of clay-slate 

 belonging to one and the same formation, shews that, during 

 the formation of these varieties, extremely similar circumstances 

 must have existed. We must not, however, extend our conclu- 

 sions to the rock termed primitive clay-slate. Its very intimate 

 connection with mica-slate prevents us from forming any other 

 opinion than that it is either a mass of pure mica, or a mixture 

 of quartz and mica. In order to draw a satisfactory conclusion 

 on this subject, it would be necessary to institute a .separate 

 investigation, for which purpose a perfect analysis of mica-slate 

 itself would be an indispensable preliminary. 



Annual Repo7-t on the State of the Useful Arts, ordered hy the 

 Society of Arts for Scotland. By Edward Sang, Esq. 

 F. R. S. E. Vice-Pres. Soc. Arts.* 



On the Progress of Exactitude in the Manufacture of Machines. 



It is a trite remark, but one that cannot be too often repeat- 

 ed, that art and science go hand in hand. " 



There are arts, the mere practice of which implies extensive 

 scientific acquirements, not perhaps in the actual operator, but in 



• Read 7th December 1830, llth January 1837, and 21st June 1837. 



