WITH THE MICROSCOPE. 199 



The action of acids and alkalies is often very valuable in render- 

 ing structures transparent, which are too opaque for examination in 

 the ordinary state. If a portion of tendon, composed of white fibrous 

 tissue, pi. XXVI, fig. 169, which is very opaque in its ordinary state, 

 be immersed in acetic acid, or in a dilute solution of potash or soda, 

 it soon becomes clear and transparent, and if the operation be 

 conducted with certain precautions, many of its original characters 

 may be brought back by subsequently neutralising the acid or 

 alkali. 



The cell wall, or rather the outer part of the cell, which is in 

 many cases too opaque to enable us to see the nucleus in the interion 

 may be made by reagents perfectly transparent so that the nucleus 

 becomes distinct and well defined. This change may be easily effected 

 by either of the reagents alluded to in the last paragraph. Albu- 

 minous textures generally may often be rendered very transparent by 

 the action of acetic acid, or by the addition of a drop of dilute 

 caustic potash or soda. 



281. Preliminary Operations. In the first place we should note 

 carefully the general characters which the substance exhibits ; its 

 form, colour, size, weight, hardness, &c. ; and fluidity, transparency, 

 tenacity, <Scc., in the case of liquids. Portions of solid textures and 

 the deposit from fluids must be subjected to microscopical examina- 

 tion, but their reaction should be always ascertained in the first 

 instance. 



2S2. Reaction. The reaction of any moist substance is found 

 out by testing it with a piece of blue, and reddened, litmus paper. 

 If the matter be dry, or the reaction of a vapour is to be tested, the 

 paper must be first moistened with a drop of distilled water. The 

 blue litmus paper is reddened by acids, and the red paper is turned blue 

 by alkalies. The reddened litmus paper is prepared by adding a 

 very small quantity of acetic acid to the infusion of litmus into which 

 it is to be dipped. 



If an acid reaction is due to the presence of carbonic acid, the 

 blue colour will be restored upon gently warming the paper upon a 

 glass slide over a lamp, or upon a warm plate. 



An alkaline reaction may depend upon the presence of volatile or 

 fixed alkali. The red colour is restored upon warming the paper 

 which has been rendered blue by the presence of volatile alkali 

 (ammonia or carbonate of ammonia), while it is not restored if the 

 change is produced by the presence of a fixed alkali (potash, soda } 

 or their carbonates, or an alkaline phosphate, &c.). 



The reaction of some objects under the microscope may be 

 ascertained by adding a little solution of litmus or of litmus slightly 



