COLLECTION OE SPECIMENS 183 



Ward's " Brown Cement " is also an admirable 

 sealing medium for a like purpose. It can be 

 obtained from any optician. When too thick from 

 age, the cement is easily thinned with methylated 

 spirit ; it dries quickly, and in a few hours' time 

 the cell can be finished off, should it be thought 

 advisable, with asphalt varnish. 



