JANUARY 25, 1884.] 
ent: the liquid is capable of dissolving solids which 
are insoluble in the vapor. The latter fact is proved 
by the experiments of Hannay and Hogarth (Proc. 
roy. soc., Oct., 1879), and also by similar experiments 
of Dr. Ramsay. A small piece of potassium iodide 
was placed in the lower part of the experimental tube, 
which was partly filled with anhydrous alcohol. The 
“upper part of the tube was free from alcohol, but its 
sides were covered with a film of crystalline potassium 
iodide. When the tube was heated and the meniscus 
disappeared, the salt in the lower part of the tube was 
dissolved, while that in the upper part remained un- 
ehanged. Similar observations were made on eosine. 
Dr. Ramsay’s second paper contains the isothermal 
lines for benzine, ether, and a mixture of benzine 
and ether, below and above the critical temperatures. 
The apparatus used resembled that of Andrews. 
The most remarkable feature of these lines is, that, 
below the critical temperature for benzine, there 
appears to be a diminution of pressure corresponding 
to a diminution of volume, immediately before com- 
plete condensation takes place. This phenomenon 
appears very slightly in a mixture of benzine and 
ether, but is not apparent in ether alone. It has 
been suggested by James Thomson (Proc. roy. soc., 
1871) that the isothermals for all gases might have 
somewhat this form below the critical temperature. 
Dr. Ramsay explains the fact by supposing that the 
molecules, when the gas has been compressed to a 
certain extent, begin to exert mutual attraction and 
relieve the pressure. The fact may be connected 
_ with the observed phenomenon that the meniscus of 
benzine remains easily distinguishable until it van- 
ishes, whereas the meniscus of ether soon becomes 
hazy. At the part of the isothermal under consid- 
eration the substance is evidently in a condition of 
unstable equilibrium, and it is difficult to see how 
this part of the curve could have been detected exper- 
imentally. 
The critical temperature and pressure of a mix- 
ture of benzine and ether were found to be not far 
removed from the mean of the critical temperatures 
and pressures of the components. 
No direct experiments have yet been made to ascer- 
tain whether heat is evolved when a gas is converted 
into liquid by pressure at temperatures above its 
critical temperature. Mr. Jamin concludes that at 
and beyond the critical point there is no latent heat. 
This conclusion, however, does not seem probable; 
since the molecular constitution of a liquid and its 
vapor are probably different, even above the critical 
_temperature. 
The conclusions which Ramsay draws from his 
experiments are summed up as follows: — 
“1°. A gas may be defined as a body whose mole- 
cules are composed of a small number of atoms. 
“2°, A liquid may be regarded as formed of ag- 
gregates of gaseous molecules, forming a more com- 
plex molecule. 
**3°. Above the critical point, the matter may con- 
sist wholly of gas if a sufficient volume be allowed, 
wholly of liquid if the volume be sufficiently dimin- 
ished, or of a mixture of both at intermediate volumes. 
SCIENCE. 99 
That mixture is, physically speaking, homogeneous 
in the same sense as a mixture of oxygen and hydro- 
gen gases may be termed homogeneous,”’ 
C. B. PENROSE. 
COLORED SKIES AFTER AN ERUPTION 
OF COTOPAXI} 
THE remarkable sunsets which have been recently 
witnessed upon several occasions have brought to my 
recollection the still more remarkable effects which I 
witnessed in 1880 in South America, during an erup- 
tion of Cotopaxi; and a perusal of your highly inter- 
esting letter in the Times of the 8th inst. has caused 
me to turn to my notes, with the result of finding 
that in several points they appear to have some bear- 
ing upon the matter which you have brought before 
the public. 
On July 8, 1880, I was engaged in an ascent of Chim- 
borazo, and was encamped on its western side at 15,- 
800 feet above the sea. The morning was fine, and all 
the surrounding country was free from mist. Before 
sunrise we saw to our north the great peak of Illiniza, 
and twenty miles to its east, the greater cone of Coto- 
paxi; both without a cloud around them, and the 
latter without any smoke issuing from its crater, —a 
most unusual circumstance: indeed, this was the 
only occasion on which we noticed the crater free from 
smoke during the whole of our stay in Ecuador. 
Cotopaxi, it should be said, lies about forty-five miles 
south of the equator, and was distant from us sixty- 
five miles. 
We had left our camp, and had proceeded several 
hundred feet upwards, being then more than 16,000 
feet above the sea, when we observed the commence- 
ment of an eruption of Cotopaxi. At 5.45 A.M. a col- 
umn of smoke of inky blackness began to rise from the 
crater. It went up straight in the air, rapidly curling, 
with prodigious velocity, and in less than a minute 
had risen 20,000 feet above the rim of the crater. I 
had ascended Cotopaxi some months earlier, and had 
found that its height was 19,600 feet. We knew that 
we saw from our station the upper 10,000 feet of the 
volcano, and I estimated the height of the column of 
smoke at double the height of the portion seen of the 
mountain. The top of the column was therefore 
nearly 40,000 feet above the sea. At that elevation it 
encountered a powerful wind blowing from the east, 
and was rapidly borne for twenty miles towards the 
Pacific, seeming to spread very slightly, and remaining 
of inky blackness, presenting the appearance of a 
gigantic inverted |_ drawn upon an otherwise per- 
fectly clear sky. It was then caught by a wind blow- 
ing from the north, and was borne towards us, and 
appeared to spread rapidly in all directions. As this 
cloud came nearer and nearer, so, of course, it seemed 
to rise higher and higher in the sky, although it was 
actually descending. Several hours passed before fhe 
ash commenced to intervene between the sun and 
ourselves; and, when it did so, we witnessed effects 
which simply amazed us. We saw a green sun, and 
1 From Nature, Dec. 27. A letter sent to Mr. Norman Lockyer. 
