On the Electrical Insulating -power of various kinds of Glass. 437 



stants contained in the three integrals are determined so as to 

 satisfy the conditions of integrability of 



udr -f- rv cos \d6 + rwd\. 



Although the foregoing solution may be far from being appli- 

 cable to the case of actual tides, still the problem is one which it 

 is necessary to be able to solve strictly before a mathematical 

 theory of such tides can be entered upon. 



Cambridge, May 23, 18/0. 



LXIV. On the Connexion between the Electrical Insulating-power 

 and Chemical Composition of various kinds of Glass. By F. L. 

 Ekman, Junior Professor at the Royal Technological Institute 

 in Stockholm*. 



IT has long been known that the electrical insulating-power 

 varies greatly in different kinds of glass ; but the causes of 

 these differences have been but little investigated. The question, 

 however, is of considerable practical importance, and I there- 

 fore, at the suggestion of Professor E. Edlund, undertook to 

 inquire whether the difference in the insulating-power of va- 

 rious kinds of glass might not be ascribed to differences in 

 their chemical composition. In this examination I employed 

 five specimens of glass, supplied to me by M. Oeller, electrical- 

 instrument maker, two of which have been found by him unfit, 

 but the remaining three good for electrical purposes. 



Most chemists, I imagine, in order to answer such a question, 

 would, like myself, in the first place direct their attention to 

 those chemical relations in the glass which can produce differ- 

 ences in the qualities of the surface. A species of glass which is, 

 within reasonable limits, rich in silica, potash, or lime ought to 

 prove a better insulator than a lime glass containing much soda, 

 simply because it is harder, and therefore preserves a perfectly 

 smooth surface better than the latter. But the insulating-power 

 of glass may depend in a still higher degree on its power to 

 resist the effect of chemical agents ; for humidity and car- 

 bonic acid in the atmosphere are at all times present to corrode 

 it. The new substances that are thus formed on the surface of 

 the glass may be themselves inferior to glass as insulators, and 

 moreover may contribute to the production of a conducting stra- 

 tum of damp upon its surface. It is well known that glass in 

 the form of powder is rapidly affected by water ; the examination 

 of glass which has lain long in the earth or in water has proved 



* Report made to the Session of December 1869 of the Royal Academy 

 of Sciences in Stockholm. Communicated by the Author. 



