700 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. 41. 



the capillaries. Wliat has been said of shellac applies equally well 

 to all the colloids. 



The same is true also of a varnish applied to a painting. It is a 

 colloid substance inflated by a volatile liquid. The latter evaporates 

 and leaves a thin porous crust of gum spread over the pamting, 

 unitmg more or less with each superficial granule of pigment of the 

 pamting, which itself is a more or less porous layer on a ground com- 

 posed of whiting and one of the ochers, united by a colloid. This 

 in turn is necessarily porous, and is spread upon a very coarse, porous 

 imen. Each layer penetrates and takes hold of the laver below. 

 The porosity of such a varnish is proved by the dark tmts of oxida- 

 tion which it presents after the lapse of time and which are con- 

 tinuous throughout the thickness of the layer of varnish. It is 

 further proven by the occasional oxidation of the oils used in the 

 paintmg below the varnish, and the chemical change of the lead 

 pigments into sulphides by the action of the sulphur dioxide con- 

 tamed m the contaminated atmosphere — all circumstances that make 

 the restoration of a masterpiece a matter of great difficulty. 



The use of gums and resins, or in fact any material that has a 

 colloidal tendency, is wholly out of the question where absolute 

 exclusion of air is demanded. Shellac is, however, a most excellent 

 binder of decaying substances, and though tlie solution may not 

 prevent air and moisture from penetrating, it is indispensable as a 

 building or bmding material. 



The production of homogeneity would demand the infiltration of 

 substances that would entirely fill up the interstitial spaces at ordi- 

 nary temperatures and be as fluent as turpentine or benzine, so as 

 to penetrate the most delicate capillaries. They should at the same 

 tune retain, if possible, some degree of firmness. To gain these 

 qualities we are obliged to have recourse to various waxes, bitumens, 

 or paraffins of the higher marsh-gas series. Stearin consists of 

 stearic acid and glycerin and contains an unsaturated molecule that 

 can not be depended upon. 



Paraffin, as is well known, is fluid when heated, and will penetrate 

 or dialyse, as it were, almost if not quite as readily as do lower mem- 

 bers of the series, such as kerosene or benzine even when cold. The 

 reason of this is that paraffin, according to Berlinerblau, contains as 

 unpurities products of the lower series, especially of a lower melting 

 pomt, which, evaporating at or below 100° C, will work their way 

 as vapor rapidly through the most delicate capillaries, then condense 

 more or less upon the walls of the latter, and furnish points of attrac- 

 tion for products of a higher melting point, until finally the more 

 slowly moving product of the highest point has penetrated the most 

 deeply seated capillaries. The greater the volatility the greater 

 also the diffusibility and power of penetration, and, on the con- 



