ON CEMENTS. 277 



but free from all rough particles ; it may be mixed up on a 

 slab with a small quantity of gold-size, and if a little litharge 

 be added, it will dry more readily. 



Besides the above described cements there are a few others 

 so useful as to require mention here. The first of these is 

 known in commerce as Suffffitt's Liquid Jet; it is said to 

 be a solution of bituminous shale in naptha: this cement 

 dries in a few minutes and is admirably suited for making 

 thin cells as well as for securing their covers, neither spirit 

 nor turpentine act upon it. It shotdd be kept tightly corked, 

 and only a small quantity exposed at a time. 



Coachmakers' Black Vaitiish may be employed as a substi- 

 tute for the above, but it does not dry quite so quickly ; this 

 can be obtained of any varnish maker ; but that sold by Mr. 

 Penney, 251, Tottenham Court Koad, is very excellent. 



Black Japan is a good cement for making thin cells by 

 Mr, Shadbolt's machine, described in page 288 ; it possesses 

 the property of becoming very hard and tough by exposure to 

 a gentle heat in an oven. The cells of Mr. Topping, also 

 described in page 288, are prepared of this material. 



In all cases where any of the above described cements are 

 used, it must be borne in mind that too much should not be 

 laid on at once, a thin coating at first, and a thicker when the 

 first is dry. 



