Xll INTKODUCTION. 



dipping, therefore, into the mercury. If the cover glass is now 

 kept down by clips, the gas chamber will be perfectly closed ; 

 and no further explanation is required to show how the gas, 

 whose effect is to be examined, may be conducted over the 

 object. 



There are certain difficulties accompanying the examination 

 of objects in gas chambers ; taking the simplest case for ex- 

 ample, a drop of blood is placed on the lower surface of the 

 cover, which is then laid on the cell, and firmly luted to it. 

 The first current of gas which passes over it dries up the edges 

 of the blood spot, and this can scarcely be avoided. It becomes 

 necessary, therefore, to experiment with great rapidity in the 

 gas chambers, or to add some indifferent fluid to the prepara- 

 tion, which may saturate the air contained in the little cell with 

 aqueous vapour without essentially altering its character. 

 But we are thus no longer working under the simplest con- 

 ditions, and due allowance must be made for this in the 

 conclusion at which we arrive. 



The employment of the moist chamber is rendered still more 

 difficult, if it be desired to warm the object whilst under ob- 

 servation with the microscope. Rollett was the first to in- 

 troduce a means of varying the temperature in microscopic 

 investigation. Max Schultze made improvements in this 

 direction, and has constructed a stage capable of being heated, 

 which can again be fitted to the stage of a microscope, is 

 capable of being warmed throughout its whole extent, and 

 can furnish the means by which the temperature of the object 

 under examination can be varied at will. Various modes have 

 since been suggested, by which the effects of elevation of tem- 

 perature upon an object can be ascertained. In Max Schultze's 

 stage, the mode of warming consists in the direct conduction of 

 heat through metal plates. The attempt was subsequently 

 made to conduct a warm fluid through the object stage, and 

 still more recently, to employ warm vapour with the same 

 object in view. A better method than any of these, and which 

 demands attention as a means of heating the stage, consists in 

 the conversion of a constant current of electricity into heat. 

 In microscopical investigation, only a very small absolute 

 quantity of heat is required, and indeed it is not necessary to 



