158 Proceedings of Royal Society of Edinburgh. [sess. 
enough for one test ; so we can make ten tests to get a good average 
in five minutes, or, let us say, a quarter of an hour for everything. 
Results Obtained with the New Apparatus. 
In my first communication on this subject, there was given a table 
in which is entered the number of particles in the atmosphere of 
this district, and the number in air polluted with the products of 
combustion. These numbers have been checked by means of the 
new apparatus, and found to he practically correct. For air from a 
Bunsen flame, different numbers have been obtained, depending on 
the manner of collecting the products; but the one entered in the 
table is not the highest that has been observed. For the air of the 
laboratory the results are very variable, hut those given in the 
table are not a bad average, and the same may be said of the 
numbers given for the outside air. 
Confining our attention now to the outside air, a great number of 
tests have been made here, and the condition of the air has been 
found to vary greatly, the smallest number observed per cubic 
centimetre is 11,000; frequently there are 50,000 to 70,000 par- 
ticles per c.c., and as many as 140,000 per c.c. have been counted. 
These numbers are very great, and one naturally asks the question, 
How many of these particles are what we might call natural, and 
what proportion is due to artificial causes ? The situation, where 
these tests were made, is what might be called “in the country.” 
It is quite outside of the town of Falkirk. It is, however, sur- 
rounded on many sides by public works, and in most directions there 
are villages at greater or less distances. It is, therefore, very evi- 
dent that the air at this situation must be very much polluted by 
artificial causes. I was constantly reminded of this while making 
my tests. At no great distance there is a railway, and when an 
engine passed, if the wind was from that direction, the particles 
would suddenly increase in number, and for a short time become so 
great it was impossible to count them. 
How that the apparatus was in a satisfactory working condition, 
it seemed desirable that the investigation should be extended, and 
tests be made of the air at other places. The first point to which 
attention was given was to find out what the number is in air under 
natural conditions — that is, free from all artificial pollution. For 
