234 Lithographic Process. 



mites, and fitted, like a bulb, to the lower end of a thermometer 

 tube. It is then filled with mercury, which rises and falls in the 

 tube, by the sensible and rapid changes that take place in the 

 contraction or dilatation of the membrane, from the humidity or 

 dryness of the atmosphere. In point of sensibility, Mr. Adie has 

 found this membrane to exceed any thing he has ever met with. 



LITHOGRAPHIC PROCESS. 



[Conchided from p. 157-] 



Lithographic chalk is a composition which can be applied to 

 the stone in a dry state like Italian chalk; and different sorts of 

 drawings may be produced with it. For the composition of this 

 chalk M. Senefelder gives a number of recipes. We extract the 

 first. Wax 4 parts; soap 6 parts ; lauip-black 2 parts. The 

 wax and the soap are melted together, then the lamp-black is 

 added ; the whole is well rubbed down on a hot iron plate, then 

 put into a saucepan and exposed to the fire until it returns to a 

 liquid state. It is then poured out on a stone plate, well im- 

 pregnated with oil, so as to form a cake of the thickness of the 

 eighth part of an inch. The mass when cooled is cut into small 

 slices, and is fit for use. In the other compositions, tallow, sper- 

 maceti, and shell lac are added in various proportions, as it is re- 

 quired to make the chalk of a harder or of a softer texture. 



There remains the composition of the printing ink, which should 

 be made of oil varnish, and fine burned lamp-black, well ground 

 and united. Sometimes ivory-black is used, and occasionally 

 Frankfort black. 



The next thing considered by M. Senefelder, is what is called 

 " the preparation of the stone ;" by which is especially meant the 

 process by which the stone receives the quality of repelling the 

 printing ink in certain distinct places. Gum arable, and some 

 other similar substances, are the principal means of thus pre- 

 paring the stone ; the operation of aquafortis or other acids only 

 rendering the stone more disposed to admit of the preparation. 

 As the gum acts onlv on the uppermost surface of the stone, and 

 by the frequent wiping ofi'of the ink in printing is gradually di- 

 minished, it becomes necessary to renew it, while the stone is 

 printing, about cwice a-day. M. Senefelder's work contains a 

 mass of minute and valuable information on this point. 



The best mode of making a steel pen for lithographic purposes 

 is then described; as also the manner of cutting sable pencils in 

 order to qualify them for the production of lines of equal thick- 

 ness. The lithographic student must also provide himself with 

 tracing and etching needles ; and with a chest well lined with 

 pitch, in which to bite in the stones. 



There are three different sorts of paper necessary. Transpa- 

 rent 



