OF CERTAIN MODERN PAPERS. 183 



retains, and this transformed into hydrochloric acid, slowly corrodes 

 the cellulose. 5th, lastly above all, the introduction, in too large apropor- 

 tion of certain mineral ingredients into the pulp, such as kaolin or china- 

 clay, pipe-clay, plaster, and chalk, substances the admixture of which 

 has for some time become general, and has had the effect, while aug- 

 menting the weight of the paper, of diminishing the proportion of fibre, 

 that is, of the rags of which it should consist. 



The quality of the paper not only depends upon the character of the 

 raw materials, but also on the mechanical preparation. For this reason 

 strong pulps always furnish a good paper, whereas tough substances, 

 roughly bruised, do not offer the same consistency. In order to obtain 

 strong pulps, they must be subjected a considerable time to the action 

 of the beating-engine, taking care not to destroy their strength by a too 

 rapid or abrupt motion of the beating-roll. The thinner the paper, 

 and therefore the more difficult to pass over the machine, the longer 

 must the pulp be kept in the beating-engine. But this operation must 

 be conducted carefully if a good and solid paper is desired. The bars 

 of the roll should rub the stuff out. If the bars are too sharp, the 

 fibre is cut and the pulp loses its cohesive element. 



We have alluded to the alteration which papers undergo from an 

 excess of chlorine, and to the traces of acid which they retain. There is 

 a very simple mode of determining the presence of these bodies in papers, 

 of which the pulp has not been sufficiently washed ; it is, to moisten 

 the sheet with a diluted solution of iodide of potassium. If there are 

 hydrochlorites in the paper, a brown spot is formed more or less dark, 

 by the action of iodine, which produces a blue colour, if the sheet has 

 been sized with starch. Moreover, these papers always give an acid 

 reaction, and an odour more or less sensible of chlorine. 



The presence of kaolin is also easily shown. A definite weight of the 

 paper, previously dried, is incinerated and the residue weighed. Paper 

 of good quality ought to leave only 2 per cent, of ash ; French filter- 

 ing paper leaves only 2 decigrammes of residue to 100 gr. of dried paper, 

 and good Swedish filtering paper (papier Berzelius) used in chemical 

 analysis, leaves, after combustion, only l-600th of its weight. Never- 

 theless, it is now very usual to meet with papers which contain l-6th 

 and even l-4th of their weight of kaolin. A sample of paper prepared 

 with glycerine, sent to me by M. Bols, a printer at Brussels, gave a 

 residue of near 30 per cent, of mineral substances, consisting chiefly of 

 silica and alumina ; ingredients which, by their excessive quantity, 

 justify the term of mineral paper being applied to this manufacture. 



Some idea of the enormous use of kaolin may be formed, when it is 

 known that M. L. Piette, in his talented " Journal des Fabricants de 

 Papier," estimated in 1854, at more than 50,000,000 kilogrammes, the 

 quantity of this substance used in the paper mills of Europe to mix 

 with rags. 



This practice, however, is hardly more new in the making of paper 



