146 Mr. H. F. Talbot on a new Property of Nitre, 



a state of fusion, and it will spread itself in a thin transparent 

 film over the surface of the glass. 



Removed from the lamp it immediately solidifies, and the 

 film in cooling cracks irregularly. As soon as the glass is 

 cool enough, let it be placed beneath the microscope (the 

 polarizers being crossed, and the field of view consequently 

 dark). We shall observe the following phaenomena. In the 

 first place the nitre appears very luminous; a proof that it is 

 in a crystalline state, and not amorphous, as (after fusion) 

 we might have expected to find it. 



For if it had lost its crystalline structure it would produce 

 no more effect when laid beneath the microscope than a thin 

 sheet of glass would do, that is to say, it would be absolutely 

 invisible, and the field of view would remain dark. 



If its crystalline structure were imperfectly restored on 

 cooling (so as to present a confused assemblage of minute 

 crystals) the field of view would appear also imperfectly and 

 irregularly luminous, according to the accidental positions of 

 the small crystals. 



Neither of these results however takes place, but an entirely 

 different one, which would hardly have been anticipated. For 

 the crystalline structure of the fused nitre is perfect ; that is to 

 say, that the film appears very luminous, and imiformly so, 

 'as if it were a thin slice which had been taken from a large 

 crystal of the substance. 



In order to see how perfectly it is crystallized, select any 

 portion of the film for observation, and turn it slowly round 

 in its own plane upon the stage of the microscope. Its 

 brightness will be seen gradually to fade, and finally, in a cer- 

 tain position, to be altogether extinguished. Now when this 

 happens it will be seen to be uniformly dark over its whole 

 surface, and the smallest irregularity in this respect would of 

 course immediately manifest itself by contrast with the rest. 

 Consequently every part of it acts together, as if it were a 

 portion of one and the same crystal. 



But this does not continue indefinitely; for if we carry the 

 eye over the whole of the dark surface which I have described, 

 and which frequently extends in breadth over a space double 

 or triple of the field of view, we shall come abruptly to its 

 boundary, and shall see it succeeded by another portion of 

 the crystalline film which is luminous. If now we darken this 

 second portion (by again turning round the glass plate to a 

 certain extent), in proportion as we do so we shall see the 

 first portion recover its light. 



Proceeding in this manner, we shall find that the whole 

 crystalline film which is spread on the glass consists of ten, 



