[11] PRESERVATION OF MICROSCOPIC MATERIALS. 617 



sulpburic acid does not, like claromic solutions, harden the object, but 

 simply kills the cells. 



" As this fluid penetrates thick chitine with difficulty, it is necessary, 

 in order to obtain good preparations of larger Isopoda, insects, &c., to 

 cut open the body and fill the body-cavity with the liquid by means of 

 a pipette. In larger objects care should be taken to loosen the internal 

 organs so that the fluid may find easy access to all parts. 



''The fluid should be applied as soon as the body is opened, so that 

 the blood may not have time to coagulate and thus bind the organs to- 

 gether. A large quantity of the fluid should be used, and it must he changed 

 as often as it lecomes turbid. The same rule holds good in the use of all 

 preservative fluids. It is well, also, especially with larger objects, to 

 give the fluid an occasional stirring up. 



"In order to avoid shrinkage in removing small and tender objects 

 from the acid to the alcohol, it is advisable to take them uj) by means 

 of a pipette or spatula, so that a few drops of the acid may be trans- 

 ferred along' with them. The objects sinking quickly to the bottom, 

 remain thus for a short time in the medium with which they are satu- 

 rated, and are not brought so suddenly into contact with the alcohol. 

 In a few minutes the diffusion is finished; and they may then be placed 

 in a fresh quantity of alcohol, which must be shaken up frequently, and 

 renewed from time to time until the acid has been entirely removed. 



"The sulphuric acid contained in this fluid causes connective tissue to 

 swell, and this fact should be borne in mind in its use with vertebrates. 

 To avoid this difficulty, Kleinenberg has recommended the addition of 

 a few drops of creosote, made from beechwood tar, to the acid. Ac- 

 cording to Dr. Mayer's experience, however, the addition of creosote 

 makes no perceptible difference in the action of the fluid. 



"This fluid must not be used with objects (<?. g., echinoderms) possess- 

 ing calcareous parts which it is desired to preserve, for it dissolves car- 

 bonate of lime and throws it down as crystals of gypsum in the tissues. 

 For such objects picro-nitric acid may be used. It is prepared as fol- 

 lows: 



Water 95 parts. 



Nitric acid (25 per cent. Na O5) 5 " 



Picric acid as much as will dissolve.* 



"Picro-nitric acid also dissolves carbonate of lime, but it holds it in 

 solution, and thus the formation of crystals of gypsum is avoided. If 

 much carbonate of linie is present, the rapid production of carbonic 

 acid (gas) is liable to result in mechanical injury of the tissues, hence, 

 in many cases, chromic acid is preferable to picro-nitric acid. 



"Picro-nitric acid is, in most respects, an excellent preservative 

 medium, and, as a rule, will be found to be a good alternative in those 

 cases where picro-sulphuric acid fails to give satisfactory results. Dr. 

 Mayer commends it very strongly, and states that with eggs contain- 



* This mixture is used undiluted. 



