1>1S 



,")() or (id , <>i" cx't'ii hoiliii^' walci', hut in the last case, the 

 object remaiiK'd in the bath I'oi" U'ss Uiaii one tiflli of a 

 second, w]u'rcii))on it was iniiru'(bately pbiiiged into cold 

 water. Immersion in mercury lieated to 40° gave good 

 ]-esubs ill tlie case of moss. Isopentane recommends itself 

 ])articnlarly in exi)eriments on })r()tozoa because of its 

 immiscil)ility with water. One can place on a thin cover- 

 glass a small drop of water containing- the protozoa, im- 

 merse tliis preparation in licinid air, then in isopentane, 

 and repeat the operation several times; the drop is still 

 intact with the organisms within it. But in spite of this 

 notable advantage, it seems, on the basis of experiments 

 reported above, that isopentane is too slow as a warming- 

 medium. 



In order to reduce the heat capacity of the preparation, 

 one must use only very thin supports. Microscope cover- 

 glasses are often too thick. Frequently w^e substituted 

 for them sheets of mica ; since these cleave easily, one can 

 get sheets which are only some ten micra in thickness. 

 Metallic foil presents the inconvenience of not being trans- 

 parent and so not allowing of microscopic observation. 



Instead of thin supports, we sometimes used, with advan- 

 tage, a ring about two millimeters in diameter, made of 

 as thin a metal wire as possible, and fastened to a light, 

 rigid rod. One simply dips this loop into the culture, and 

 thus obtains, in the thin film within the ring, quite a con- 

 siderable number of organisms. With the diameter of 

 loop indicated, it is only seldom that the tilm breaks either 

 when immersed in liquid air or when immersed in the 

 warming medium. We often found it advantageous to 

 reduce the thickness of the wire still further by flattening 

 it with a hammer. 



With protozoa we also employed another method which 

 consisted in placing- the culture in an atomizer and spray- 

 ing it thence into liquid air. But the freezing of the w^ater 

 which encloses the organisms liberates heat and retards 

 cooling. 



We likewise tried to emulsify the culture with oil. When 



