Vlll INTRODUCTION. 



Any imperfections in this respect, however, will increase with 

 the capacity of the space by which the object is surrounded. 

 It should therefore be made as small as practicable, and, if pos- 

 sible, altogether dispensed with ; in other words, where practi- 

 cable, only a covering glass should be used, the edges of which 

 are oiled. The pressure which this exercises on the object is 

 unimportant, and may, indeed, easily be avoided altogether ; 

 for it is only requisite to form an outside wall with oil, and to 

 place a small quantity of the fluid within the space thus en- 

 closed, before applying the covering glass, in order to protect 

 it entirely from the weight of the latter. Circumstances may 

 exist, however, which render it necessary that the preparation 

 should be exposed to the air. It may, for instance, be requisite 

 to ascertain the influence of various gases; in these cases a 

 chamber must be used, of as small a size as possible, except 

 where some special arrangements are made, enabling the 

 amount of watery vapour present to be regulated. I employ 

 for this purpose a ring of putty, varying in thickness according 

 to circumstances ; the object is then to be attached, as usual, 

 to the lower surface of the covering glass ; this is now to be 

 brought down upon the ring of putty, and to be gently pressed 

 down on the object with the handle of the scalpel. A drop of 

 water placed upon the slide is sufficient to saturate the space 

 with aqueous vapour, and to prevent the object from drying. 

 Great caution must, however, be used ; for it will be found that 

 the dry, smooth, polished covering plate becomes immediately 

 tarnished when it is placed on the wall of putty. The drop of 

 fluid should therefore only have a small surface, in order that 

 it may not evaporate to too great an extent, and, on the other 

 hand, it should not be too small, lest the object dry with too 

 great rapidity. It is obvious that small variations in the pro- 

 portion of water in the object are unavoidable. 



A moist chamber formed in this fashion can easily be con- 

 verted into a so-called gas chamber. In that part of the soft 

 wall of putty which corresponds to the middle line of the glass 

 slide, a small glass tube is to be introduced on each side, and 

 to these caoutchouc tubes can be attached, which can be closed 

 by bull-dog forceps when the passage of gas is not required. 

 But when gases are to be transmitted, the necessary communi- 



