NATURAL PHILOSOPHY. 203 



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adopted, nor have they evidence that any method which has been adopted, 

 will not produce fading pictures unless such precautions are taken. 



Causes of Fading. The most common cause of fading has been the presence 

 of hyposulphite of soda, left in the paper from imperfect washing after 

 fixing. 



The committee think it right to state, that they have been unable to find 

 any test to be relied upon, which can be used to detect a minute portion 

 of hyposulphite of soda, hi the presence of the other substances which are 

 obtained by boiling photographs hi distilled water and evaporating to dry- 

 ness ; yet they have no doubt of the truth of the above statement, from the 

 history given of the mode of washing adopted. 



The continued action of sulphureted hydrogen and water will rapidly 

 destroy every land of photograph ; and as there are traces of this gas at all 

 times present hi the atmosphere, and occasionally in a London atmosphere 

 very evident traces, it appears reasonable to suppose that what is effected 

 rapidly in the laboratory with a strong solution of the gas, will take place 

 also slowly but surely in the presence of moisture, by the action of the very 

 minute portion in the atmosphere. 



The committee find that there is no known method of producing pictures 

 which will remain unaltered under the continued action of moisture and the 

 atmosphere of London. 



They find that pictures may be exposed to dry sulphureted hydrogen gas 

 for some tune with comparatively little alteration, and that pictures in the 

 coloration of which gold has been used, are acted upon by the gas, whether 

 dry or in solution, less rapidly than any others. 



They also find that some pictures which have remained unaltered for years, 

 kept hi dry places, have rapidly faded when exposed to a moist atmosphere. 

 Hence it appears that the most ordinary cause of fading may be traced to the 

 presence of sulphur, the source of which may be intrinsic from hyposulphite 

 left in the print, or extrinsic from the atmosphere, and hi either case the 

 action is much more rapid hi the presence of moisture. 



Mode of Mounting Photographs. The committee find that, taking ecoial 

 weights, dried at a temperature of 212, of the three substances most frequently 

 used, viz. gelatine, gum, and paste, the latter attracts nearly twice as much 

 moisture as either of the former, and as hi practice a much smaller weight of 

 gelatine is used than of gum, gelatine appears to be the best medium of these 

 three ; and the committee have evidence of fading having in some cases been 

 produced by the use of paste. 



In illustration of some of the circumstances alluded to above, the committee 

 think it well to mention some instances of prints at present in their posses- 

 sion: Out of several prepared together in 1844, three only were unaltered, 

 and these were varnished soon after their preparation, with copal varnish. 

 Half of another print of the same date was varnished, and the other half 

 left ; the unvarnished half has faded, the varnished part remains unaltered. 

 Three pictures were prepared in 1846, all at the same time, with the same 

 treatment; when finished, one was kept unmounted, the other two were 

 mounted with flour paste at the same tune, one of these latter having been 



