c o 
by Mr. Walker, are concentrated nitrous acid two parts 
by weight, water one part; or this mixture, cooled, to 
the temperature of the atmofphere, eighteen ounces ; ot 
Glauber-’s fait, a pound and a halt" avoirdupois; and ot 
fal-ammoniac, twelve ounces. On adding the Glauber’s 
fait to the nitrous acid, the thermometer fell 52°, viz. 
from 50 to—2 ; and on the addition ot the fd-ammoniac, 
it fell to—9 0 . Thus Mr. Walker was able to freeze 
quickfilver without either ice or fnow, when the thermo¬ 
meter flood at 45 0 ; viz, by putting the ingredients in 
four different pans, and inclofing thefe within each other. 
See Chemistry, p. 384, 385. 
Exceffive degrees of cold occur naturally in many parts 
of the globe in the winter feafon. Even in the Eaft 
Indies, cold is fometimes exc.effive. It is mentioned in 
Mr. Pennant’s View of Hindooftan, publifhed in 1798, 
that a froll has been experienced at Delhi, which lafted 
three nights. All the tanks were completely frozen over, 
and veffels of brafs and copper, filled with water, burft or 
fpl it a funder. 
* Although the thermometer in this country hardly ever 
defcends fo low as o, yet in the winter of 1780, Mr. Wil- 
fon of Glafgow obferved, that a thermometer laid on the 
fnow funk to 25 0 below o ; and Mr. Derliam, in the year 
1708, obferved in England, that the mercury flood with¬ 
in one-tenth of an inch of its flation when plunged into 
a mixture of fnow and fait. At Peterfburg, in 1732, the 
thermometer flood at 28° below o; and when the French 
academicians wintered near the polar circle, the thermo¬ 
meter funk to 33 0 below o: and in the Afiatic and Ame¬ 
rican continents, Hill greater degrees of cold are often, 
obferved. 
The effefts of thefe extreme degrees of cold are very 
furprifing. Trees are burtt, rocks rent, and rivers and 
lakes frozen feveral feet deep : metallic fuhflances bliiter 
the fkin like red-hot iron : the air, when drawn in by 
breathing, hurts the lungs, and excites a cough : even 
the effects of fire in a great meafure feem to ceafe ; and 
it is obferved, that though metals are kept for a conlider- 
able time before a flrong fire,' they will ltill freeze water 
■when thrown upon them. When the French mathema¬ 
ticians-wintered at Tornea in Lapland, the external air, 
when fuddenly admitted into their rooms, converted the 
moifture of the air into whirls of fnow; their breafls 
feemed to be rent when they breathed it, and the contaft 
of it was intolerable to their bodies; and the fpirit of 
wine, which had not been highly reftified, burfl fome of 
their thermometers by the congelation of the aqueous part. 
Extreme cold too often proves fatal to animals in thofe 
countries where the winters are very fevere; thus 7000 
Swedes perifhed at once in attempting to pafs the moun¬ 
tains which divide Norway from Sweden. But it is not 
necelfary that the cold, in order to prove fatal to human 
life, fiiould be fo very intenfe as has juft been mentioned ; 
it is only requifite to be a little below 32 0 of Fahrenheit, 
or the freezing point, accompanied with fnow or hail, 
from which Ihelter cannot be obtained. The fnow which 
falls upon the clothes, or the uncovered parts of the 
body, then meits, and by a continual evaporation carries 
off the animal heat to fuch a degree, that a fufficient 
quantity is not left for the fupport of life. In fuch cafes, 
the perfon firft feels liimfelf extremely chill and uneafy ; 
he turns liftlefs, unwilling to walk or ufe exercife to keep 
himfelf warm, and at laft turns drowly, fits down to re- 
frefh himfelf with fleep, but wakes no more. 
Cn the cold felt at the top of high mountains, and at 
certain depths under ground, the celebrated Euler, in his 
Letters to a German Princels, has fuggefted the following 
explication : “ It appears very furprifing, that we fhould 
feel the fame degree of cold in all regions, after we have 
xifen to a certain height, fay 24,000 feet; confidering that 
the variations with rel’peft to heat, on the earth, not only 
in different climates, but in the fame country, at different 
feafons of the year, are fo perceptible. This variety, which 
L D. 767 
takes place at the furface of the gfobe, is undoubtedly oc- 
cafioned by the fun. It appears, at firft fight, that his in¬ 
fluence mull be the fame above and below, efpecially when, 
we reflect, that a height of 24,000 feet, or a German mile, 
which is 4000 fathoms, or ibinewhat under 4-jj-ths miles 
Englifn, though very great with refpeft to us, and even 
far beyond the height of the loftieft mountains, is a meie 
nothing compared to the diilance of the fun, which is about 
thirty millions of German miles. This is, therefore, a very 
important difficulty, which we mull endeavour to folve. 
For this purpofe I begin with remarking, that the rays of 
the fun do not communicate heat to any bodies, but fuch 
as do not grant them a free paffiige. Bodies, through, 
which we can difcern objefts, are denominated tranfpa- 
rent, pellucid , and diaphanous. Thefe bodies are glafs, 
cryftal, diamond, water, and feveral other liquids, though 
fome are more or lefs tranfparent than others. One of 
thefe tranfparent bodies being expofed to the fun, is not 
heated to fuch a degree as a body not tranfparent, as 
wood, iron, See. Bodies not tranfparent are denominated 
opaque. A burning-glafs, for example, by tranfmitting 
the rays of the fun, lets on fire opaque bodies, while the 
glafs itfelf is not fenfibly heated. Water expofed to the 
fun becomes fomewhat warm, only becaufe it is not per¬ 
fectly tranfparent; when we fee it conliderably heated by 
the fun at the brink of rivers, it is becaufe the bottom, 
being an opaque body, is heated by the rays which the 
water tranfmits. Now, every heated body communicates 
that heat to all adjoining bodies ; the water accordingly 
derives heat from the bottom. If the water be very deep, 
fo that the rays cannot penetrate to the bottom, it has no 
perceptible heat, though the fun bears upon it. 
“ As air is a very tranfparent body, to a much higher 
degree than glafs or water, it follows, that it cannot be 
heated by the fun, becaufe the rays are freely tranfmitted 
through it. The .heat which we frequently feel in the 
air, is communicated to it by opaque bodies, which the 
rays of the fun have heated; and, were it poflible to an¬ 
nihilate all thefe bodies, the air would fcarcely undergo 
any change in its temperature by the rays of the fun: 
expofed to it or not, it would be equally cold. But the 
atmofphere is not perfectly tranfparent; it is even fome¬ 
times fo loaded with vapours, that it lofes almoll entirely 
its tranfparency, and prelents only a thick fog. When 
the air is in this Hate, the rays of the fun have a more 
powerful influence upon it, and heat it immediately. But 
thefe vapours rile to no great height; at the height of 
24,000 feet, and beyond, the air is fo fubtile and lo pure, 
that it is perfectly tranfparent; and, for this reafon, the 
rays of the fun cannot immediately produce any effeft 
upon it. This air is likewife too remote from terreftrial 
bodies, to receive a communication of heat from them; 
they aft only upon l’uch as are adjacent. Hence we eafily 
perceive, that the rays of the fun cannot produce any ef¬ 
feft in regions of the air very much elevated above the 
furface of the earth; and that the fame degree of cold 
mull always, and univerfally, prevail in fuch regions, as 
the fun has no influence there, and as the heat of terref¬ 
trial bodies cannot be communicated fo far. This is 
nearly the cale on the fummit of very high mountains, 
where it is always much colder than on plains and in val- 
lies. There are clouds, however, above thefe mountains, 
and in almoft as great a quantity as above the plains, 
which is demonftrated by the fnows which cover the high- 
eft fummits. There are few naturalifts who have not been 
furprifed by clouds in their excurfions upon the moun¬ 
tains. The heat that is felt when fuch clouds are formed, 
mull be attributed almoft entirely to the tranfmilfion of 
the water which found itfelf diflbived in the air, under 
the form of elaltic fluid, to a liquid Hate. The heat of 
the loiar rays, intercepted by the cloud, can produce no 
change in the inferior temperature, as it would have been 
tranimitted from the ground. 
“ The city of" Quito, in Peru, is almoft under the equa- 
1 tor a , 
