38 OQuertes. 
found it fail except by some awkwardness of my own. ‘The 
difficult point is to know exactly when to press down the cover- 
glass. Be sure to have at hand some iron or stone slab to cool 
the slide rapidly. Fossi/ done, in which the lacune, etc., are often 
filled up with mineral matter, may be mounted in the usual way. 
R. SSE. 
Queries. 
All Questions and Answers should be clearly and concisely written on one side 
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Correspondents, in sending us answers to questions, are desired to commence 
by quoting the number and title of the question as printed, before beginning their 
reply. All answers should reach us before the 12th of the month. 
325.—Weight of the Earth.—Will some one tell me how the 
earth is weighed, and its density and mass computed ? GEO. 
326.—Preparing Rock Sections.—Have any readers of the 
Linguirer succeeded in preparing good sections of rocks, for the 
microscope, z7zthout the aid of the lathe and diamond powder, 
and if so how? Or are these requisites essential for success? Also, 
is there any safe way of transferring the sections to a fresh slide 
when finished, or otherwise preventing the scratches, which dis- 
figure the slip on which the specimen has been ground down. 
G. Eee 
327.—Cultivation of Sundew.—I should be glad to hear of 
any one who has been successful in cultivating plants of Drosera 
rotundifolia ; how they may be grown so as to live for more thana 
short time. They generally seem to die very soon after transplant- 
ing, or live for at most a-year. Pinguicula vulgaris, on the other 
hand, can be readily grown in a pot of wet earth, and may be easily 
reared from the seeds, which it produces freely under cultivation. 
GH: 
328.—Preserving Entomological Specimens.—Another en- 
quiry on this well-worn subject may perhaps seem unnecessary, 
put amid all the information given in various books and. journals 
as to the best way of saving insects from the attacks of mites and 
larvze, I have been unable to find exactly the information required. 
Thus camphor is excellent for the purpose, but camphor tends to 
produce greasiness in certain butterflies. Naphthaline is harmless, 
alike to the specimens in the cabinet, and the mites or larve feeding 
upon and destroying them. Benzine requires constantly being 
