Intelligence and Miscellaneous Articles. 167 



there is no irradiation of light from behind, at least not nearly so much 

 from the spaces as from the white ground. At all events, whatever 

 the cause may be, I have found the lines with the spaces give a much 

 better and sharper impression. 



The picture of the lines should be a positive one, and very clear. 

 I found the collodion prepared with the iodide of iron, according to 

 the formula given in this Magazine, July 1854, to act admirably. It 

 must be very sensitive, on account of the smallness of the aperture 

 necessary for the required sharpness. 



I have no doubt that much finer lines than these I have got might 

 be obtained by the same process. 



Your obedient Servant, 



Thomas Woods, M.D. 



ON THE BOILING OF LIQUIDS. BY M. L. DUFOUR, 



The ebullition of liquids, instead of taking place under normal 

 circumstances of temperature and pressure, varies, as is well known, 

 with the vessel in which the liquid is contained. In an earthen 

 vessel, for instance, the ebullition is at a higher point than in one of 

 metal; and Marcet has shown that the treatment the glass has 

 experienced, washing with sulphuric acid, &c, often modifies the 

 boiling-point to the extent of several degrees. Water deprived of 

 air and placed in the conditions of a water-hammer, may be heated 

 several degrees above 100° C. without passing into the gaseous state, 

 but it then boils violently. Donny has shown that water free from 

 air and carefully heated, may be raised to 135° without assuming 

 the gaseous state. This retardation of ebullition is further found in 

 other liquids ; and the violent production of vapour is a frequent in- 

 dication of it in glass vessels. 



The ebullition not being produced except at a temperature higher 

 than that at which the elastic force of the vapour is equal to the ex- 

 ternal pressure, is due to two causes, — first, the adhesion of the liquid 

 to the substance of the vessel ; and secondly, the absence of air in 

 solution. 



There are nevertheless some curious cases in which the retardation 

 of boiling cannot be explained by the adhesion to a solid, and the 

 absence of air, yet where the contact of a solid produces a sudden 

 formation of vapour. If linseed oil be heated in a dish to 105° or 

 110° and a few drops of water be allowed to fall, they will sink to 

 the bottom of the vessel. The moment they touch there is a sudden 

 formation of vapour ; the globule of water, slightly diminished, is 

 repelled a few millimetres from the bottom ; it again sinks, giving 

 rise to a fresh disengagement of vapour, which raises it again, and 

 so on. There is no perceptible evaporation from the globules of 

 water so long as they float on the oil, and are not in contact with 

 the side of the vessel ; and it is only on the sudden contact of the 

 solid that a bubble of vapour is suddenly produced. It is natural to 

 inquire what would take place if the water during its being heated 



