ASTONISHING PROPERTIES OF GLASS. 785 
prey, flock together in February to choose their mates. They soon 
disperse, and are not seen afterwards, but in pairs. 
Pairing is unknown to quadrupeds that feed on grass. To such it 
would be useless, as the female gives suck to her young while she is 
feeding. If M. BufFon deserves credit, the roe-deer are an exception. , 
They pair, though they feed on grass, and have but one litter in a 
year. Beasts of prey, such as lions, tigers, wolves, pair not. The 
female is left to shift for herself and her young ; which is a laborious 
task, and often so unsuccessful as to shorten the life of many of them. 
Pairing is essential to birds of prey, because incubation leaves the 
female no sufficient time to hunt for food. Pairing is not necessary 
to beasts of prey, because their young can bear a long fast. Add 
another reason, that they would multiply so fast by pairing, as to 
prove troublesome neighbours to the human race. Among animals 
that pair not, males fight desperately abotit a female. Nor is it 
unusual for seven or eight lions to wage bloody war for a single 
female. 
The same reason that makes pairing necessary for gregarious birds, 
obtains with respect to gregarious quadrupeds, those especially who 
store up food for winter, and during that season live in common. Dis- 
cord among such would be attended with worse consequences than 
even among lions and bulls, which are hot confined to one place. 
The beavers, with respect to pairing, resemble birds that place their 
nests on the ground. As soon as the young are produced, the males 
abandon the stock of food to their mates, and live at large, but return 
frequently to visit them while they are suckling their young. Hedge- 
hogs pair, as well as several of the monkey kind. We are not well 
acquainted with the natural history of these animals ; but it would 
appear that the young require the nursing care of both parents. 
Seals have a singular economy. Polygamy seems to be a law of 
nature among them, as a male associates with several females. The 
sea-turtles have no occasion to pair, as the female performs her task 
at once by laying her eggs in the sand. The young are hatched by 
the sun, and immediately crawd to the sea. 
Astonishing Properties of Glass. 
1, Glass is one of the most elastic bodies in nature. If the force 
with which glass balls strike each other, be reckoned 16, that where- 
with they recede by virtue of their elasticity will be nearly 15.2. 
When glass is suddenly cooled, it becomes exceedingly brittle; and 
this brittleness is sometimes attended with very surprising phenomena. 
Hollow balls, made of annealed glass, with a small hole in them, will 
fly to pieces by the heat of the hand only, if the hole by which the 
internal and external air communicate be stopped with a finger. 
Lately, however, some vessels made of such annealed glass have been 
discovered, which have the remarkable property of resisting very hard 
strokes given from without, though they shiver to pieces by shocks 
received from the fall of very light and minute bodies dropped into their 
cavities. These glasses may be made of any shape, but their bottoms 
must always be thicker than their sides. The thicker the bottom is, 
5 G 
