Prof. J. P. Cooke on Lecture Experiments* 



55 



the volume has disappeared. As this, however, would require a 

 deep cistern of mercury, and as the interior surface of the tube 

 is left in such a very dirty condition as to obscure the result, we 

 prefer to transfer the tube to a jar of water, which, when the 

 cap is removed, at once displaces the mercury and washes out 

 the interior without sensibly impairing the accuracy of the ex- 

 periment. 



Water. — The points to be illustrated in the case of water, or 

 rather of free steam, are all indicated by the equation 



+ 







= H 2 oJ 



There is no more striking experiment in this connexion than 

 the decomposition of water by sodium, but as made in the usual 

 way the experiment is attended with no little danger. Sodium 

 in certain states explodes violently when brought into contact with 

 water ; and after several accidents of this sort we have substituted 

 the solid sodium-amalgam above described with the best results. 

 We place an ounce or more of this amalgam in a common gas- 

 flask and pour upon it water. The action is very moderate and 



must be assisted by a gentle heat; but on 

 heating the flask with a gas-lamp the gas is 

 evolved rapidly and with great regularity. 

 We collect it over the pneumatic trough, and 

 illustrate its properties in the usual way. 



We illustrate the fact that water is com- 

 posed of two volumes of hydrogen to every 

 one of oxygen by the electrolysis of dilute 

 sulphuric acid, using for the purpose the 

 decomposing-cell (fig. 1) already described. 

 We collect the gases in two tall bell-glasses 

 standing side by side on the shelf of the same 

 pneumatic trough. These bells are of course 

 calibrated, and the equal divisions marked by 

 rubber rings. Six cells of Bunsen's battery, 

 medium size, give a rapid evolution of gas. 



Lastly, we illustrate the fact that, when 

 the elementary gases unite to form aqueous 

 vapour, three volumes are condensed to two, 

 by means of the eudiometer represented in 

 fig. 7. This apparatus is easily made with 

 a common iron casting, well known to steam- 

 fitters as a "return bend." Any machinist 

 will easily adapt this casting to the required 

 use by attaching to it, as shown in the figure, 

 a circular iron plate to serve as a stand, by 



Fig. 7. 



