72 THE BOOK OF THE LANTERX. 



the cylindrical form, and rubbsd down with a file ; or a 

 piece of good hard chalk will serve, if there is nothing 

 better at hand. The following mixture has been recom- 

 mended as one from which a hard substance can be 

 moulded which will take the place of the usual lime 

 cylinder : 



Precipitated Chalk 4 parts. 



Heavy Magnesia, Carbonate 1 part. 



Mix to paste with gum-water, and mould to form. 



Of late years some new limes have been introduced, with 

 the trade-mark "Excelsior." These will do admirably for 

 the blow -through jet, but will not (at any rate in my 

 hands) withstand the attack of a powerful mixed jet. I 

 have always regretted that I cannot use them, for they are 

 uniform in size, are accurately turned and bored, and are 

 packed in a. very convenient manner. A good hard 

 material which will last for several hours, and which is not 

 affected by damp, is a thing that is much wanted for lime- 

 light working. The following extract from Lieut. Drum- 

 mond's paper in the Philosophical Transactions, 1826, is 

 interesting, as showing that the discoverer of the lime-light, 

 made trials of various substances, but found lime to be the 

 best : 



" The results of several trials made at the commencement, 

 gave for 



Lime 37 times 



Zirconia 31 times 



Magnesia 16 times 



the intensity of an Argand burner. The oxide of zinc was 



