MATHEMATICS AND NATURAL PHILOSOPHY. 163 



forms* — is evidently at the close of the series, as a transition-group to the Rudistes. 

 This position, and the general arrangement of the families and genera as given 

 above, has been adopted by the -writer in his lectures during the last three or four 

 years. E. J. C. 



MATHEMATICS AND NATURAL PHILOSOPHY. 



MOSEE S IMAGES AND A KE"W ACTION OF LIGHT. 



In the year 1842, Professor Moser, of Kcenigsberg, called attention to an inter- 

 esting class of phenomena, to which his name has since been attached, under the 

 title of "Moser's Images," although Dr. Draper, of New York, had, two years pre- 

 viously, announced his observation of similar facts. If a wafer or piece of money 

 is laid on a plate of glass, and the glass breathed upon ; then, after the breath 

 has evaporated, and the object has been removed from the plate, although the eye 

 can detect no trace, a renewed breathing will cause the spot where it rested to 

 become visible. If the coin be breathed upon and laid upon the plate, and a few 

 seconds be suffered to elapse, on removing the coin, although as before no trace 

 can be perceived, on breathing on the plate, the image of the coin will be produced 

 in minute detail. At first M. Moser was inclined to attribute these effects to 

 differences of temperature, as he found that on placing the coin on a heated silver 

 plate, the same effect was produced ; and, also on reversing the experiment by 

 placing a heated coin on a cold plate. He also found that, instead of the breath, 

 the vapors of mercury, iodine, chlorine, and the like could be used ; and thus he 

 drew the general conclusion that " when a polished surface is put in contact with 

 a body of different temperature, it acquires the faculty of condensing on portions 

 of itself all kinds of vapors, and of fixing them either by adhesion or che- 

 mical combination." He however abandoned this hypothesis as too limited, 

 and announced that mere contact, independently of difference of temperature, was 

 sufficient to produce the effect, and he was thus able to imitate the action of light 

 on the Daguerrian plates in profound darkness, and obtained images of various 

 objects, laid on an iodised silver plate, in darkness, bj afterwards subjecting the 

 plate to mercurial vapor, or to the sunlight. Extending his researches, he found 

 that the iodising of the plate was not essential to the photographic process of 

 Daguei-re ; and that sunlight was capable of making an impression on pohsbed plates 

 of different kinds, which could be reproduced at pleasure by subjecting them to 

 various vapors. Thus, placing a black screen out of which figures had been cut, 

 before a well polished silver plate, and exposing it to sun-light, the figures were 

 perfectly defined when the mercurial vapor was applied to the plate. When a 

 plate of glass was similarly exposed, the breath alone was sufficient to render the 

 figures visible, and they could be made to reappear at pleasure, even after a very 

 long interval of time. Hence M. Moser inferred that " light acts on all substances 

 and its action can be rendered visible by aid of any vapor which adheres or can 

 be chemically combined with the substance," and of this general proposition the 



* Some Zoologists place the Brachiopoda higher in the scale than the Lammellibranchiata 

 The gradual assumption, however, of foot and branchise in the development of the latter 

 would appear to be opposed to this view. 



