WITH THE MICROSCOPE. 59 



little camphor), corrosive sublimate, persulphate of iron, sulphate of zinc, 

 and solutions of several other salts, have been recommended as pre- 

 servative solutions, but although adapted for the preservation of 

 animal substances, they cannot be employed for microscopical 

 specimens, in consequence of their tendency to render the textures 

 very opaque and granular. Mr. A. E. Verrill (Siliman's Journal, 

 March 1865) recommends a solution made with nitre, rock salt, and 

 arseniate of potassa. My own experience, however, has led me to 

 discard all solutions containing salts for microscopical purposes. 



112. Arseiiious Acid has been much recommended, and 

 Dr. Andrew Clarke has preserved many specimens of lung tissue 

 and other structures in an aqueous solution of this substance. 



113. Arseniuretted hydrogen gas has also been recommended 

 for the preservation of animal substances, but it is not adapted for 

 microscopical preparations. Dr. Richardson has lately kept animal 

 substances from decomposition by immersing them in an atmosphere 

 of nitrogen, which is prepared by placing a piece of phosphorus in 

 a stone jar containing common air, and provided with an air-tight 

 cover. The oxygen is soon exhausted, and no decomposition can 

 take place. 



Most of the preservative solutions which I have referred to may 

 be obtained of Mr. Highley, Green Street, Leicester Square. The 

 mode of using them will be described further on. Every micro- 

 scopist engaged in any special enquiry will of course alter the com- 

 position of these solutions in any way experiment may show to be 

 advisable. Great improvements doubtless may yet be made in many 

 preservative solutions. A series of exact, experiments of the effects 

 of the different fluids upon the same textures is much to be desired, 

 and this is one of the questions upon which amateurs might con- 

 tribute most valuable information. 



CELLS FOR PRESERVING MICROSCOPICAL SPECIMENS. 



All objects intended for microscopical observation should be pro- 

 tected with a cover of thin glass. This cover prevents the entrance 

 of dust, and protects the object from exposure to the atmosphere. 

 The fluid in which many objects are placed for examination would 

 rise in vapour which would condense upon the object-glass, and give 

 rise to great inconvenience were it not prevented from evaporating 

 by a thin glass cover. If the thin glass, however, should press upon the 

 object placed upon the glass slide, its distinctness would in many 

 cases be impaired, or the structure might be entirely destroyed an 

 inconvenience which may be prevented by placing some insoluble sub- 



