3«] I SI se6ls, that conceal themselves un- der ground hi a toq)id state during the winter, and enjoy the genial vai-mth of" the spring, again to disappear at a season when the heat of the atmosphere was much higher than on their first emerging from their subterraneous abode. From these considerations, Dr. Darwin infers, that such plants as are sheltered in a warm room, dur- ing the winter, ought to be occa- sionally exposed to a cooler atmos- phere, in order to increase their ir- ritability ; otherwise their growth in the succeeding spring will be much retarded. For the same rea- son, the continual vicissitudes of the air and weather are essentially necessar)' to mankind ; as the fre- quent changes of heat and cold have an obvieus tendency to pre- serve or restore their irritability, and consequently the a6tivity of the system. Hence not only the health and energ}'^of menare more conspi- cuous in our variable climate, but their longevity is comparatively greater tlian on those tropical con- tinents, which experience both a more considerable degree of heat, and a more steady atmosphere. IscHURY, See Urine. ISINGLASS, or IchthyocoUa, a preparation from different species of a Russian fish, called sturgeon, of which we have given some ac- count in the article Caviare. It may also be produced from the air-bladders of the cod, or gadvs, as well as from those of other fish inhabiting fresh waters. The method of making isinglass was for ages kept a secret with the Bussians, but has lately been dis- covered : we extract the following process, from the 63d vol. of the Philos(yphicalTransa6iio7is : — First, ' the sinewy parts c£ the fish are- I S I boiled in water, till they are cfis- solved; then the viscid liquor is strained, and suffered to cool. When cold, the fit is cari^fuliy taken off, the liquor again bo.ledto a due con- sistence, then cut in pieces and rolled into a semi-circular twist, irt which state they are suspended on a string, till carefully dried. The sounds or air-bladders of fresh-water fish, in general, arc the most transparent, flexible, and delicate substances, and conse- quently furnish the finest isinglass. But the intestines and pcritoneufn of the fish constitute inferior sorts of this article; denominated hooh, and ordinary staple. The Iclugas, being the largest and most plenti- ful fish in the Russian rivers, jield the greatest quantity of this animal glue, which, on account of its strength, is preferred to all other kinds. Isinglass is most successfully pre- pared in the summer ; as frost changes its colour, deprives it of weight, and impairs iis gelatinous principle ; but the forms into which it is twisted by the Russians, arc useless, and frequently injurious to its native qualities. These pe- culiar shapes were probably adopt- ed with a view to conceal the real substance, and thus to preserve the monopoly. The Newfoundland and Iceland fishermen split open the fish, as so^i as they are taken, and throw the back bones, with the sounds annexed, into a heap ; but, before putrefa6tioncomniences, the sounds are cut out, washed and salted for use. In performing this operation, the best, namely, the intercostal parts, are left behind ;-.— the Iceland fishermen arc so sensible of this circumstance, that they beat the . bone upon a block witlx a iliick Slick,
