52 



Prof. J. P. Cooke on Lecture Experiments. 



dealers in philosophical apparatus in Boston. A much simpler 

 decomposing-cell, although not quite so efficient an apparatus, 

 can be made with a common U-tube supported on a wooden 

 stand and mounted with rubber stoppers, delivery-tubes, and 

 platinum electrodes in every respect like the other. 



The second point to be illustrated in regard to hydrochloric 

 acid is, that it consists of equal volumes of its constituent gases ; 

 and although by our first experiment, if made in the way we 

 have described, it is shown that the gases are evolved in nearly 

 equal proportions, yet a second experiment is required to esta- 

 blish the absolute truth* of this important fact. The apparatus 

 we use for this purpose is represented in fig. 5, and the details 



Fig. 5. Fig. 6. 



i^ 



of the several parts are shown in fig. 6. The two gases are 



* We here of course leave out of view the small difference which arises 

 from the fact that the gases do not absolutely obey Mariotte's law, and are 

 therefore unequally condensed even by the atmospheric pressure. The 

 volumes of the two gases would undoubtedly be exactly equal under greatly 

 diminished pressure ; but under the pressure of the air the volume of the 

 molecular weight of chlorine is about y^-g-iless than the corresponding volume 

 of the molecular weight of hydrogen, a difference by no means inappreciable 

 in accurate experiments. 



