THE METEOROLOGY AND CLIMATE OF PLYMOUTH. 439 
concluded that ozone was oxygen condensed to one-half its volume; 
but Professor Roscoe says it is condensed in the proportion of 3 to 2, 
and can be reconverted into ordinary oxygen gas again by the 
application of great heat. Professor Tyndall in his researches on 
radiant heat (Mode of Motion, p. 339) showed that ozone must be 
atoms of oxygen packed in swinging groups; ¢.e. forming mole- 
cules, not atoms; and it is now generally accepted that three 
atoms of oxygen go to form a molecule of ozone. It is cha- 
racterized by its high oxydizing powers, and it readily decomposes 
most organic compounds as well as iodide of potassium, turning 
the potassium into potash, and liberating the iodine. This pro- 
perty has been seized on for the purpose of making ozonometers ; 
for starch when acted on by free iodine gives a very characteristic 
blue colour, light or deep, according to the amount of iodine 
liberated. Hence paper soaked in starch which has been wetted 
with a solution of iodide of potassium is the ozonometer generally 
in use, the paper after exposure being compared with a standard 
set of colours settled on. I have kept no record of the presence 
of ozone in Plymouth, because the true amount cannot yet be 
determined by any known process easy of application ; and hence 
what are called ozonometers should rather be called ozonoscopes, 
as they show only the probable presence of ozone, and not the 
amount in the atmosphere. The causes for this are as follows: 
(1) There is no method which can be introduced into general 
use for regulating the quantity of air which passes over the papers, 
and yet give free exposure; and if we cannot determine the 
quantity of air which has given the colour, it is certain we cannot 
estimate the amount of ozone in the atmosphere. 
(2) Other substances in the air, especially the trioxide of nitro- 
gen, produce a similar reaction on the test papers, so that we are 
not sure that any discolouration which may take place is due 
entirely to the presence of ozone. 
(3) No discolouration takes place if the test papers be quite 
dry, and it is owing to the hydroscopic nature of the papers that 
any change of colour is visible in them. Hence when the air is 
moist we get better returns than when it is dry. 
Still for comparisons at the same place observations by means of 
ozonoscopes may be useful. From its great oxydizing powers this 
forms, from a health point of view, one of the most useful in- 
gredients our atmosphere contains; all disease germs are at once 
