METALLIC STAINS 209 



objects thoroughly penetrated by light from all sides during the 

 process of reduction. Objects, therefore, should always be so 

 thin that light can readily stream through them. He im])regnates 

 for a few hours in 1 per cent, gold chloride (§ 400) in the dark, 

 then brings the objects ivithout icashing out rvith ivater, the gold 

 solution being just superficially mopped up with blotting-paper, 

 into 1 per cent, formic acid. They are to be set up in this, in a 

 tube or otherwise, so that the light may come through them from 

 all sides, and exposed to diffused daylight in summer, or direct 

 sunlight in winter, for six to eight hours ivithout a break. They 

 must not he moved about more than can be helped in the acid. 

 If the acid becomes brown it may be changed for fresh. The 

 temperature of the acid should not be allowed to rise over 20° C, 

 whence direct sunlight is to be avoided during the summer. He 

 mounts in glycerin or his syrup (§ 382). He finds such preparations 

 absolutely permanent. 



POST-IMPREGNATION 



408. Gerlach's Method (Stricker's Handh., 1872, p. 678) : 

 Spinal cord is hardened for fifteen to twenty days in a 1 to 2 

 per cent, solution of bichromate of ammonia. Thin sections 

 are made and tlirown into a solution of 1 part of double chloride 

 of gold and potassium to 10,000 parts water, which is very slightly 

 acidulated with HCl, and after ten to twelve hours are washed in 

 hydrochloric acid of 1 to 2 : 3000 strength, then brought for ten 

 minutes into a mixture of 1 part HCl to 1000 parts of 60 per cent, 

 alcohol, then dehydrated and mounted in balsam. 



See further, for Nerve Centres, under " Nervous System." 



409. GoLGi {Mem. Accad. Torino [2], xxxii, 1880, p. 382) puts 

 tissues previously hardened in 2 per cent, solution of bicliromate 

 of potash for ten to twenty minutes into 1 per cent, solution of 

 arsenic acid, then into h per cent, solution of chloride of gold and 

 potassium for half an hour, washes in water, and reduces in sun- 

 light in 1 per cent, arsenic acid solution, which is changed for 

 fresh as fast as it becomes brown. Mount in glycerin. Sunny 

 weather is necessary. 



410. Apathy's Method {Zeit. wiss. Mik., x, 1893, p. 349 ; 

 Mitth. Zool. Stat. Neapel, xii, 1897, p. 729) : The material to be 

 used must have been fixed either in sublimate or in a mixture of 

 equal parts of saturated solution of sublimate in 0-5 per cent, salt 

 solution and 1 per cent, osmic acid (this more particularly for 

 Vertebrates). The material should be imbedded as quickly as 

 possible, either in paraffin or in celloidin. Sections are made and 

 fixed on slides, and after the usual treatment with iodine, etc., 

 are either put into distilled water for from two to six hours, or are 

 rinsed in water, treated for one minute with 1 per cent, formic 

 acid, and again well washed with water. 



