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472                     TALBOT'S OBSERVATIONS ON FUSED NITRE                    [4
colour is not always uniform over the whole plate, indicating a varial thickness. But this condition of things is not permanent. After perhaps quarter of an hour, the plates break up in a surprising fashion and the gene: appearance is totally changed.
Moreover the transformation may be accelerated. "Let a film of fus nitre be obtained in the manner already mentioned, and let it be allowed cool during three or four minutes. The plate of glass should be turned rou upon the stage of the microscope until the crystalline film is darkened accurately as possible. Things being thus adjusted, let the observer ton. the film with the point of a needle, while he is observing it in the microsco] He will perceive that the touch immediately produces a luminous spot on t dark surface, and this spot will slowly expand itself in all directions li a luminous wave. This is a very curious object, but difficult to describ And further on " If however we touch it prematurely, as, for instance, duri the first minute after it has become solid, this change does not'take place."
I have made a few trials to ascertain whether the life of the plates can prolonged.    Protection from atmospheric moisture did little good.    Anotli plate kept for five hours at a temperature not much short of that of boili: water  was  found to  have   undergone  transformation.    But, as  might expected, a higher temperature over a diminutive gas flame acted as a sa: guard, and the plate after removal behaved like one newly formed.
I have found that nitre may be replaced by chlorate of potash, with t advantage that the plates will keep (at any rate in an artificially warm room) for weeks and perhaps indefinitely. The appearances are similar b less beautiful, as colour is riot so often developed. The chlorate is me fusible than nitre, and the heat should not be pushed beyond what is need for fusion.
Other salts, for example silver nitrate, which fuse in the anhydrous sta without decomposition may also be employed, as is probably known to the who prepare objects for the microscope. But Talbot's early observations ' nitre are rather special and deserve recall as they seem to be but little kno-wrresponded fully to the suppositions of theory.