346 JOHN AITKEN ON 
effect produced by any substance placed in the tube was due to that substance 
alone. The result of heating the clean and empty tube was most remarkable, 
and very unexpected. A slight heating was sufficient to give rise to a very 
dense fog, on admission of steam to the receiver. We might have imagined 
that the careful washing the tube received was sufficient to make the glass 
clean. Yet we see it was still so foul that heat drove off sufficient matter in a 
fine state of division as to give rise to a dense fog. The glass tube was now 
highly heated, to see if heat would cleanse it. After cooling it was again 
heated to the same amount as at first. It was now found to be quite inactive. 
No fogging whatever appeared in the receiver. If, however, the tube was again 
highly heated fogging appeared. In testing different substances placed in the 
tube, it was therefore necessary to use only a low degree of heat, so that none 
of the effect might be due to the tube. After each experiment the tube was 
highly heated, to thoroughly cleanse it, before introducing the substance to be 
tested. When this was done, and a lower degree of heat employed, I could 
perfectly trust to the tube being inactive. 
The next experiment was made with a small piece of brass wire placed in 
the testing tube. While it was cold there was of course no fogging, but when 
slightly heated, a dense clouding resulted. A piece of iron wire, and other 
substances, all gave a similar result. The wires were now highly heated in a 
Bunsen flame before being put in the testing tube. On heating they were 
now found to be quite inactive, not the slightest fogging appeared. The high 
temperature had acted on them as it acted on the glass, and destroyed their 
dust-producing powers. 
A piece of brass wire was now carefully filed bright, so as to remove all 
uncleanness from it, it was then placed in the experimental tube, care being 
taken that it was not touched with the hands. When heated it only gave 
rise to the faintest cloudiness. These experiments prove that the cloudiness 
was produced by some matter driven off by the heat from the outside of the 
metal. The slight cloudiness produced by the filed wire being due to the 
slight contamination got when being filed. 
_ The amount of matter which is driven off these wires by heat is extremely 
small, and its result as a fog-producer so great, that this apparatus places in 
our hands a means of detecting in gases quantities of matter so small as 
almost to rival in delicacy the spectroscope. The following experiment will 
give an idea of the marvellous smallness of the amount of matter which may be 
detected in this way. If we take a small piece of fine iron wire, 73> of a grain 
in weight, and place it in the experimental tube, and apply heat, it will give 
rise to a very decided cloudiness. Now take the wire out, and if you so much 
as touch it with your fingers, on again returning it to the tube and heating, 
the fact of your having touched the +3, of a grain of iron wire will be declared 
