GENERAL METHODS OF INVESTIGATION, BY S. STRICKER. XIX 



temperature of each part of the cover changes as the object 

 glass sweeps over it, and must necessarily vary within certain 

 limits, even with the best means of regulating the temperature. 

 Each time that it is cooled, a precipitation of the watery vapour 

 from the atmosphere that is saturated with it must occur. 

 Recklinghausen and Kiihne have endeavoured to obviate this 

 inconvenience by the construction of a more complicated appa- 

 ratus for supplying heat. Whilst the results of these ex- 

 periments are still unknown, it is advisable to postpone the 

 investigations on the effects of heat in the gas cells. The 

 reason that has induced me to describe the construction of the 

 heatable gas cell at so great a length is, that it affords excellent 

 results in quite another line of inquiry. If the floor of the 

 cell be covered with a drop of water, and the preparation is 

 attached to the under surface of the cover over the water, all 

 increase of temperature will cause the atmosphere within the 

 cell to contain more watery vapour, of which a part will con- 

 dense on the object. If a delicate test object be examined, 

 such, for example, as the blood corpuscles constitute for a 

 practised observer, it will be remarked that every addition to 

 the temperature produces a perceptible alteration in the object, 

 attributable to the increased proportion of water in the serum. 

 We thus possess the power of supplying water, in very precise 

 proportions, to preparations enclosed within a cell. 



It has been further ascertained that the action of gases on 

 blood is different in accordance with the amount of water that 

 it contains. The results of the experiments that have been 

 hitherto made will be detailed in the chapter on the blood. A 

 single example may, however, here be given to show the 

 advantage that gas cells capable of being heated can afford. 

 It may, in some cases, be very desirable to be able to vary the 

 temperature within certain limits with rapidity. I have, in- 

 deed, had occasion to perform some experiments in which it 

 was requisite to pass, alternately, iced water and steam through 

 the cell. For this purpose I have constructed a metal slide, in 

 which a central perforation (c, fig. vili.) permits the passage of 

 light ; and the preparation may again either be placed upon a 

 glass cover cemented down, or may be so arranged that the 

 hole in the slide serves as a cell. The plate itself must consist 



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