W. A. Miller on Photographic Transparency, etc. 103 
ful radiant in the presence of space, and with no screen above it 
check its radiation. Into space it pours its heat, chills itself, conder 
and the tropical torrents are the consequence. The expansion of the 
air, no doubt, also refrigerates it; but in accounting for those deluges, 
the chilling of the vapor by its own radiation must play a most import- 
ant part. The rain quits “the ocean as vapor ; it returns to it as water, 
wasted = radiation —_ apap Similar remarks rhe to the cnanh of 
our latitudes. The warmed air, charged with — rises in columns, 
50 as to penetrate the wan screen whic ugs earth; in the pres- 
ence of space, the head of each pillar wastes it heat by radiation, con- 
denses to a cumulus, ne constitutes the visible capital of an invisible 
column of saturate 
mbe 
e 
unendurable ; in Sahara the dryness of the air is sometimes such, that 
though during the day “ the soil is fire and the wind is flame,” the chill 
at night is painful to bear. In Australia, also, the thermometric range 
is enormous, on account of the absence of this qualifying agent. A 
clear day, and a dry day, moreover, are very different things. The 
atmosphere may possess great visual clearness, while it is charged with 
aqueous vapor, and on such occasions great chilling cannot occur by 
terrestrial radiation. Sir John Leslie and others have been perplexed 
ise different metals were in a saad degree imilan if not ‘detical, Sub- 
sequent investigations have, however, shown him that the absorbent ef- 
fects of the bisulphid upon the chemical rays are so great, that the con- 
clusions then drawn from observations made by this sh medium 
tion. Notwithstanding the great length 
of the chemical spectra obtained by the aid of the bisulphid, not more 
_ than one-sixth or or one-seventh of the true extent of the spectrum produced 
q by the electric spark betwe is procured, as may shown 
