ZOOLOGY AND BOTANY, MIOHOSCOPY, ETC. 543 



changed. The proper degree of hardening is reached in from two 

 weeks (in warm weather) to seven weeks (in cold weather). The 

 second step is to immerse the hardened pieces in a • 75 per cent, 

 solution of silver nitrate for twenty-four to forty-eight hours. The 

 room in which this silver process is carried on must be kept loell 

 warmed. 



The black staining is successively imparted to the axis-cylinders 

 of the nerve-fibres, the ganglion-cells, and, lastly, the neuroglia-cells. 

 When the black staining is attained, and this is verified by ex- 

 amining a few trial sections in glycerin, the pieces are placed in 

 alcohol, frequently changed, until the alcohol remains clear, in order 

 to remove all traces of the silver nitrate. This must be done 

 effectually, otherwise the specimens will not keep. The treatment 

 preparatory to mounting in dammar, which is preferable to Canada 

 balsam, consists in washing several times in absolute alcohol, trans- 

 ferring to creosote, and in clearing up in oil of turpentine or in oil 

 of origanum. The sections are to be preserved in dammar without the 

 imposition of a cover- glass. The author mounts his specimens on 

 large cover-glasses and then adjusts them in a wooden frame or slide 

 with a window, so that the sections are kept quite free from dust, and 

 can also be examined from both sides. It is of course necessary to 

 preserve the mounted specimens in a dark place. 



The disadvantages of the method, says Prof. Golgi, are the length 

 of time required to obtain the requisite reaction, the uncertainty 

 arising from the varying periods necessary to produce the proper 

 degree of hardness, and the different conditions in which the different 

 layers of the same piece are frequently found. These disadvantages 

 are modified by : — (a) Copious and frequent injections of either a 2i per 

 cent, solution of bichromate, or a similar solution in which five or six 

 grammes of gelatin have been dissolved. The injection may be made 

 with an ordinary syringe or a siphon apparatus, through the aorta or 

 carotid, (fe) By hardening with bichromate at a constant temperature. 

 In an incubator, maintained at a temperature of 20°-25° C, the 

 reaction point was reached in eight or ten days, (c) By hardening in 

 equal j^arts of Erlicki's and Miiller's fluid, the necessary consistence 

 was obtained in five to eight days. 



The second method consists in hardening the pieces in a mixture 

 of bichromate and osmic acid, followed by immersion in silver 

 nitrate. It may be applied as follows : by immersing small pieces 

 of quite fresh nervous tissue in the following mixture : — of potassium 

 bichromate 2 to 2^ per cent, solution, eight parts ; of osmic acid 

 1 per cent, solution, two parts. Having been transferred to the 

 silver nitrate solution, as in the first method, the black reaction is 

 found to begin on the second or third day, and to be completed by the 

 tenth or twelfth. But in this method the pieces must be allowed to 

 remain in the silver nitrate until they are wanted for section, allowing 

 two days for soaking in alcohol. Although this treatment gives 

 sufficiently good and rapid results, it is better to place the pieces in 

 the bichromate solution for two to thirty days, and then change to 

 the mixture of osmic acid and bichromate, and afterwards in due 



