102 Dr. A. Waller's Observations on certain 



Many bodies possess this property, but the mineral steatite, 

 or soap-stone, produces the effect better than any other I know. 

 A considerable degree of friction may be used over the traces 

 thus produced by steatite, without affecting the appearance of 

 the traces when breathed upon repeatedly. The glass may 

 even be heated considerably without affecting them. By ex- 

 amining with the microscope the parts that have been traced 

 upon by steatite, we are unable, any more than with the naked 

 eye, to detect any material cause for the deposition of vapours 

 in these places, as it probably depends upon the transparency 

 of the mineral, which being so attenuated is unable to affect 

 the rays of light. When the traces have been brought out by 

 breathing upon them, they must be covered with another piece 

 of glass, which impedes the evaporation of the water and allows 

 them to be submitted to the microscope. The parts untouched 

 by the steatite present the appearances that have been already 

 mentioned. On the lines created by the mineral, the drops 

 of water are differently disposed, their long diameters being 

 parallel to the direction of the lines. These minute drops 

 very much resemble the globules of gas deposited from a 

 liquid, the only difference between the two consisting in the 

 deviation from the globular form in the liquid traces, which 

 evidently arises from the power which the water possesses of 

 wetting glass. 



It is evident, therefore, that the secondary cause of these 

 images is a difference in the position of the minute drops of 

 water, reflecting the light differently from the other drops, 

 which are irregularly disposed on the other parts of the glass. 



There exists another method of fixing vapours, which has 

 been long known, and to which I believe attention was first 

 directed by Prof. Draper. It consists in merely placing a 

 body on a plain surface, such as that of a metallic speculum, 

 or even of glass ; after a short time it is found that simple 

 contact, such as this, has caused some molecular action, as the 

 spot occupied by the object will become apparent by breathing 

 on it in the same way as with the images of steatite. This 

 observation is the more interesting, as it serves as a connect- 

 ing link between the effects of mechanical power and those 

 caused by other agents. 



The experiments of Mr. Hunt have shown the influence of 

 heat in causing the fixation of vapours. 



An image of this sort formed on glass by the breath, when 

 examined under the microscope, presents exactly the same ap- 

 pearance as those formed by steatite. The same difficulty is 

 experienced in bringing out, by mercurial vapours, the ther- 

 mographic images on glass, as is found with the traces of 



