1879.] the Effect of Heat on the Di-iodide of Mercury. 285 



than twice as dense as that of mercury. The vapour condenses to 

 rhombic prismatic crystals, which frequently become scarlet while 

 cooling, but which, if they still remain yellow when cold, instantly 

 become scarlet if rubbed or otherwise mechanically agitated. Accord- 

 ing to Warington this change is due to the transformation of the 

 rhombic prisms into acute square-based octohedrons with truncated 

 summits. If the yellow prismatic crystals are placed under the 

 microscope, and are then touched, the change to the red variety may 

 be observed to go on through the mass of contiguous crystals, accom- 

 panied by a slight movement, but the external form of the crystals 

 remains unchanged, consequently pseudomorphous crystals are pro- 

 duced, and the larger rhombic prisms have been resolved into a mass 

 of minute octohedrons. Frankenheim asserts that by the application 

 of a very gentle heat, both the red and the yellow crystals may be 

 sublimed together, and he believes that the vapour of the yellow 

 crystals passes off at a lower temperature than that of the red. 

 Warington found that the precipitate produced by iodide of potas- 

 sium in chloride of mercury, appeared under the microscope to be 

 composed of rhombic lamina?, which gradually altered their form by 

 the truncation of the edges, until they disappeared, while square- 

 based octohedrons were produced in their place. 



The iodide is clearly capable of existing in two crystalline forms 

 belonging to different systems, and of passing from the one form to 

 the other, either by diminution of temperature or by simple 

 mechanical means. Such a substance would seem to be likely to 

 possess peculiarities in its modes of expansion under the influence of 

 heat. In order to test this the iodide was submitted to the same 

 experimental treatment as that employed in the case of the iodide of 

 silver, and previously described in detail. 



Homogeneous rods of the iodide of mercury were heated in paraffine 

 in the expansion apparatus described and figured in the previous paper, 

 and the extent of expansion due to a given range of temperature was 

 noted. The apparatus was standardised by means of a rod of fine 

 homogeneous silver. The same micrometer, reading to . 1 th of an 

 inch, was employed, and the mode of conducting the experiments was 

 precisely the same as in the case of the iodide of silver. Two slight 

 changes were made in the apparatus however : — the one consisted in 

 the substitution of a massive stone base for the wooden one hitherto 

 used ; and the other the replacement of the glass rods moving in 

 stuffing boxes, by curved equal-armed levers moving over the rim of 

 the trough, by which means the leakage of hot paraffine at the stuff- 

 ing boxes was prevented. 



Bars of the iodide of mercury were cast in clean glass tubes, and 

 here at the outset the experimental difficulties commenced. For not 

 only was it difficult to obtain a homogeneous rod, on account of the 



