XXIV INTRODUCTION. 



down on the soft putty. The cell being now complete, the 

 electric current is conducted by the strips of metal to the 

 object, through which it passes at the same time; this lies 

 immediately beneath the cover, and can therefore be examined 

 with the highest powers. It is, moreover, no small advantage 

 to combine the application of electricity with researches on 

 the influence of gas, because we can neutralize or aid the 

 effects of the current by the introduction of different gases. 



On breaking the current, heat is developed in the tissue. I 

 have measured the amount thus set free in my arrangement of 

 the induction current, and find that it amounts, when the core 

 is fully thrust down, to about 3 C. (5 Fahr.) If an uncovered 

 drop of blood is under examination with strong ordinary lenses, 

 these become dimmed at the instant of the passage of the cur- 

 rent, but after a short period they again become clear. The 

 preparation, however, very soon dries up. It is requisite in such 

 cases to determine what are the effects of the sudden elevation of 

 temperature, and what are those of the electric current alone. 

 An additional means of research consists in effecting a change 

 in the fluid components of a microscopic object. We have not 

 as yet been able to succeed in combining this mode of investi- 

 gation with the application of gases. A reliable experiment in 

 which an alteration in the fluid is effected is only practicable 

 when the object is placed between the slide and the cover, the 

 borders of which at two opposite points at least have not been 

 oiled. To one of these points a strip of filtering paper with 

 sharply cut edges should be attached, and at the other the fluid 

 which is to be applied may be introduced by a small tube, one end 

 of which has been drawn out into a long point. When the strip 

 of filtering paper is attached to the side of the cover, it sucks 

 up the fluid of the preparation : a current is immediately es- 

 tablished, which as a general rule carries everything off that is 

 not firmly adherent. If a little time is now allowed to elapse, 

 it is possible by the cautious application of a very small strip 

 to cause a slow and feeble current to pass over the superficies 

 of the preparation whilst the deeper part remains at rest. If 

 at any time the fluid is altogether withdrawn, the cover sinks 

 until the deepest layers of the solid elements which cling to 

 the cover are pressed flat, unless, indeed, they are too resistant to 



