208 Mr. Talbot's Account of the 



to bear sunshine, it may be afterwards itself employed as an 

 object to be copied ; and by means of this second process 

 the lights and shadows are brought back to their original 

 disposition. In this way we have indeed to contend with the 

 imperfections arising from two processes instead of one; but I 

 believe this will be found merely a difficulty of manipulation. 

 'I propose to employ this for the purpose more particularly of 

 multiplying at small expense copies of such rare or unique en- 

 gravings as it would not be worth while to re-engrave, from 

 the limited demand for them. 



I will now add a few remarks concerning the very singular 

 circumstance, which I have before briefly mentioned, viz. that 

 the paper sometimes, although intended to be prepared of the 

 most sensitive quality, turns out on trial to be wholly insensible 

 to light, and incapable of change. The most singular part of 

 this is the very small difference in the mode of preparation 

 which causes so wide a discrepancy in the result. For instance, 

 a sheet of paper is all prepared at the same time, and with the 

 intention of giving it as much uniformity as possible : and yet, 

 when exposed to sunshine, this paper will exhibit large white 

 spots of very definite outline, where the preparing process 

 has failed ; the rest of the paper, where it has succeeded, turn- 

 ing black as rapidly as possible. Sometimes the spots are of 

 a pale tint of ccerulean blue, and are surrounded by exceed- 

 ingly definite outlines of perfect whiteness, contrasting very 

 much v^ith the blackness of the part immediately succeeding. 

 With regard to the theory of this, I am only prepared to state 

 as my opinion at present, that it is a case of what is called 

 " unstable equilibrium." The process followed is such as to 

 produce one of two definite chemical compounds; and when 

 we happen to come near the limit which separates the two 

 cases, it depends upon exceedingly small and often impercep- 

 tible circumstances, which of the two compounds shall be 

 formed. That they are both definite compounds, is of course 

 at present merely my conjecture ; that they are signally diffei- 

 ent, is evident from their dissimilar properties. 



I have thus endeavoured to give a brief outline of some of 

 the peculiarities attending this new process, which I offer to 

 the lovers of science and nature. That it is susceptible of 

 great improvements, I have no manner of doubt ; but even in 

 its present state I believe it will be found capable of many use- 

 ful and important applications besides those of which I have 

 given a short account in the preceding pages. 



