296 Proceedings of the Royal Physical Society. 



rocks, pumice and manganese nodules, volcanic debris, 

 minerals, sands, clays, and oozes. On examination of a 

 great many of these specimens, it was found impossible to 

 determine their structure accurately without making trans- 

 parent sections of them ; and on account of their extreme 

 friability this was found quite impossible by the usual 

 method of preparing rock sections, namely, by the lapidary's 

 wheel. It was therefore necessary to find some method of 

 rendering them hard and compact, so that they could be sub- 

 jected to this process. To attain this end, experiments were 

 made with various substances, and after many failures and 

 much labour a method was devised, which has proved suc- 

 cessful, and will be of great service to mineralogists, geolo- 

 gists, and others in investigating the composition of soft 

 rocks, sands, etc. 



I have received Mr Murray's permission to describe the 

 method finally adopted. The principle of the method con- 

 sists in the introduction of some foreign substance to cement 

 the grains together, and make the material to be examined 

 hard and compact, before it can be ground or cut into thin 

 and transparent sections. The method is carried out by 

 soaking the substances of which sections are desired in a 

 solution of gum copal in ether, and then evaporating the 

 ether — a method which, I understand, is in use by some 

 naturalists for making sections of the hard parts of Echino- 

 derms. 



PKEPARA.TION AND USE OF THE CeMENT. 



The first process consists in preparing a solution of gum 

 copal in ether. Take one-half pound of the best gum copal 

 and place it in a strong glass jar, sufficient to hold about one 

 quart, which must have a finely ground stopper, so as to 

 make it perfectly air-tight, otherwise the ether will quickly 

 evaporate and the gum copal will be wasted; add to the 

 gum copal about 20 fluid ounces of ether B.P., the specific 

 gravity of which is '735. This composition should stand for 

 at least two days, during which time it should be shaken 

 frequently, or stirred with a glass rod ; when all the gum 

 copal is dissolved, it should form a clear, thin, transparent 



