RADIATION THROUGH THE EARTH’S ATMOSPHERE. 
82 
scraped in quantities sufficient to form a small snowball. The beam of the electric 
lamp, moreover, was sent through a large receiver placed on an air-pump. A single 
stroke of the pump caused the precipitation of the aqueous vapour within, which 
J| became beautifully illuminated by the beam; while, upon a screen behind, a richly- 
coloured halo, due to diffraction by the little cloud within the receiver, flashed forth. 
The waves of heat speed from our earth through our atmosphere towards space. 
These waves dash in their passage against the atoms of oxygen and nitrogen, and 
against the molecules of aqueous vapour. Thinly scattered as these latter are, we 
might naturally think meanly of them as barriers to the waves of heat. We might 
imagine that the w T ide spaces between the vapour molecules would be an open door for 
the passage of the undulations ; and that if those waves were at all intercepted, it would 
be by the substances which form 99£ per cent, of the whole atmosphere. Three or 
four years ago, however, it was found by the speaker that this small modicum of aque¬ 
ous vapour intercepted fifteen times the quantity of heat stopped by the whole of the 
air in which it was diffused. It was afterwards found that the dry air then experi¬ 
mented which was not perfectly pure, and that the purer the air became, the more it 
approached the character of a vacuum, and the greater, by comparison, became the 
action of the aqueous vapour. The vapour was found to act ■with 30, 40, 50, 60, 70 
times the energy of the air in which it was diffused; and no doubt was entertained 
that the aqueous vapour of the air which filled the Royal Institution theatre, during 
the delivery of the discourse, absorbed 90 or 100 times the quantity of radiant heat 
which was absorbed by the main body of the air of the room. 
Looking at the single atoms, for every 200 of oxygen and nitrogen there is about 
one of aqueous vapour. This one, then, is eighty times more powerful than the 200 ; 
and hence, comparing a single atom of oxygen or nitrogen with a single atom of aque¬ 
ous vapour, we may infer that the action of the latter is 16,000 times that of the former. 
This was a very astonishing result, and it naturally excited opposition, based on the 
philosophic reluctance to accept a result so grave in consequences before testing it to 
the uttermost. From such opposition a discovery, if it be worth the name, emerges 
with its fibre strengthened ; as the human character gathers force from the healthy 
antagonisms of active life. It was urged, that the result was on the face of it impro¬ 
bable ; that there were, moreover, many ways of accounting for it, without ascribing 
so enormous a comparative action to aqueous vapour. For example, the cylinder 
which contained the air in which these experiments were made, was stopped at its ends 
by plates of rocksalt, on account of their transparency to radiant heat. Rocksalt is 
hygroscopic; it attracts the moisture of the atmosphere. Thus, a layer of brine readily 
forms on the surface of a plate of rocksalt; and it is well known that brine is very 
impervious to the rays of heat. Illuminating a polished plate of salt by the electric 
lamp, and casting, by means of a lens, a magnified image of the plate upon a screen, 
the speaker breathed through a tube for a moment on the salt; brilliant colours of thin 
plates (soap-bubble colours) flashed forth immediately upon the screen—these being 
caused by the film of moisture which overspread the salt. Such a film, it was con¬ 
tended, is formed when undried air is sent into the cylinder ; it was, therefore, the 
absorption of a layer of brine which was measured, instead of the absorption of aqueous 
vapour. 
Tliis objection was met in two ways. Firstly, by showing that the plates of salt 
when subjected to the strictest examination show no trace of a film of moisture. 
Secondly, by abolishing the plates of salt altogether, and obtaining the same results 
in a cylinder open at both ends. 
It was next surmised that the effect was due to the impurity of the London air, 
and the suspended carbon particles were pointed to as the cause of the opacity to 
radiant heat. This objection was met by bringing air from Hyde Park, Hampstead 
Heath, Primrose Hill, Epsom Downs, a field near Newport in the Isle of Wight, 
St. Catherine’s Down, and the sea-beach near Black Gang Chine. The aqueous 
vapour of the air from these localities intercepted at least seventy times the amount 
of radiant heat absorbed by the air in which the vapour was diffused. Experiments 
made with smoky air proved that the suspended smoke of the atmosphere of West 
London, even when an east wind pours over it the smoke of the city, exerts only a 
fraction of the destructive powers exercised by the transparent and impalpable aqueous 
vapour diffused in the ah. 
