HALF-AN-HOUR AT THE MICROSCOPE, 179 



the hardening be done. Brain, spinal cord, and kidney are ren- 

 dered sufficiently hard for cutting in three weeks ; lung, liver, 

 spleen, pancreas, intestines, tongue, etc., take a longer time, 

 usually from five to eight weeks. A saturated aqueous solution of 

 picric acid is sometimes used as a hardening agent, but its action 

 has a very persistent yellow dye, which is much against its 

 employment for this purpose. 



Another method for Hardening the Tissues.— After injection I 

 place the object at once in equal parts of alcohol and water ; 

 allow to remain for some hours, so that the gelatine becomes 

 solid. If carmine mass has been used, alcohol and water is the 

 only suitable fluid for hardening, and a few drops of acetic acid 

 should be added, to prevent diffusion of carmine. If Prussian 

 blue, either alcohol, Miiller's fluid, or picric acid may be used. 

 Some recommend a ^ per cent, solution of osmic acid. But it 

 must only be used for small pieces, as it does not penetrate far. 

 For those readers who wish for further information, I would recom- 

 mend Robin's " Du Microscope et des Injections," Frey's " Das 

 Mikroskop," etc., "Journal of Royal Micros. Soc," etc. 



Ibalf'^'an^^lbour at tbe flDicroecope, 



Mitb /I1M\ Uutfen Mest, f .X.S., jf.lR./ID.S., etc. 



Plates i8, 19, 20. 



.fficidium compositarium, var. C. Tussilaginis, being, as it 

 should be, mounted to show both surfaces of the leaf, we are 

 enabled to gain a view both of the " Cluster-cups " on the ufider 

 surface, and of the " Spermogones " on the tipper. 



How the contents of the latter, representing male organs, 

 situated on one side of a leaf, gain access to the former, on the 

 opposite side, is a question deserving of careful thought and 

 observation. I am inclined to think from what I have seen that 

 it is through the agency of insects. The ^xidium of the lesser 

 Crowfoot is very abundant here in its season, about April ; at such 

 time " sun and showers " are proverbial, and I have often noticed 

 flies about on the bright, shining leaves. Now, it is easy to 

 suppose that the viscid substance containing the " spermatia " 

 may adhere to their feet, and so be carried to the "Cluster-cups" 



J 



