On the Sources of the Nitrogen of Vegetation. 525 
ammonia by passing through sulphuric acid, and then over pumice- 
stone saturated with sulphuric acid. It then passed through a solu- 
tion of carbonate of soda before entering the apparatus enclosing the 
plant, and it passed out again through sulphuric acid. 
Carbonic acid, evolved from marble by measured quantities of 
hydrochloric acid, was passed daily into the apparatus, after passing, 
with the air, through the sulphuric acid and the carbonate of soda 
solution. 
The enclosing apparatus consisted of a large glass shade, resting in 
a groove filled with mercury, in a slate or glazed earthenware stand, 
upon which the pan, with the pot of soil, &c., was placed. Tubes 
passed under the shade, for the ingress and the egress of air, for the 
supply of water to the plants, and, in some cases, for the withdrawal 
of the water which condensed within the shade. In other cases, the 
condensed water was removed by means of a special arrangement. 
One advantage of the apparatus adopted was, that the washed air 
was forced, instead of being aspirated, through the enclosing vessel. 
The pressure upon it was thus not only very small, and the danger 
from breakage, therefore, also small, but it was exerted upon the 
inside instead of the outside of the shade; hence, any leakage would 
be from the inside outwards, so that there was no danger of unwashe 
air gaining access to the plants. 
The conditions of atmosphere were proved to be adapted for 
healthy growth, by growing plants under exactly the same circum- 
stances, but in a garden soil. The conditions of the artificial soil 
were shown to be suitable for the purpose, by the fact that plants 
grown in such soil, and in the artificial conditions of atmosphere, 
developed luxuriantly, if only manured with substances supplying 
combined nitrogen. 
Passing to the subjects of collateral inquiry, the first question con- 
sidered was, whether plants growing under the conditions stated 
would be likely to acquire nitrogen from the air through the medium 
of ozone, either within or around the plant, or in the soil; that bod 
oxidating free nitrogen, and thus rendering it assimilable by the plants. 
Several series of experiments were made upon the gases contained 
in plants or evolved from them, under different circumstances’ of 
light, shade, supply of carbonic acid, &e. When sought for, ozone 
was in no case detected. The results of the inquiry in other re- 
spects, bearing upon the points at issue, may be briefly summed up 
as follows :— 
1. Carbonic acid within growing vegetable cells and intercellular 
passages suffers decomposition very rapidly on the penetration of 
the sun’s rays, oxygen being evolved. 
2. Living vegetable cells, in the dark, or not penetrated by the 
direct rays of the sun, consume oxygen very rapidly, carbonic acid 
being formed. 7 
3. Hence, the proportion of oxygen must vary greatly according 
to the position of the cell, and to the external conditions of light, and 
it will oscillate under the influence of the reducing force of carbon- 
matter (forming carbonic acid) on the one hand, and of that of the 
