u INCUB 



thit i A mistake ; fat oil ought to rise in asbestine as 

 well as in other wicks. Yt this would only be gaining 

 an jncombiutible wick. The ancient, however affirm, 

 thaT azbestot once heated never cools, which has pro- 

 bably been the source of other opinions that it may 

 comtitute a perpetual lamp. Set Pliny Hist. Natural. 

 lih. xix. cap. 4. xxxvi. cap. 31; DioMorides, lib. v. cup. 

 J58 ; ludorus Origiaum. lib. xvi. cap. 4; Kirchcr, Muu- 

 dn* S^kUrraneiu, voL ii. var. loc.; Cetlius Rhodiginus 

 1.; M. Pauli Veneti de Kegi- 



omibut QnentalHiiu, ap. Gryttaia Kttviu Orbit, p. 355 ; 

 Pancirollu* dt Helms dt Pcrloitit; Philosophical Trans- 

 actioitf, vol*. xiv. xv. xxii. xxvii. li. Bell's Travels, 

 voLi. 



INCUBATION is that instinctive process by which 

 bird* sit unremittingly upon their eggs, with the ex- 

 ception of short intervals employed in quest of food, un- 

 til, by imparting their own vital 'warmth, the young ac- 

 quire* sufficient strength, and breaks the shell. 



Persona not conversant with natural history have be- 

 lieved, that life does not exist in the egg until the com- 

 mencement of incubation, or the application of artificial 

 boat. The effect of either does no more, however, than 

 unfold the latent germ. About 10 V of heat are requir- 

 ed to hatch the eggs of our common domestic poultry, 

 which, in hens, must be continued during 21 daya. 

 Sometimes the period is longer or shorter, ranging be- 

 tween 19 and 23. The period required by the eggs of 

 other birds is extremely diversified ; whence Count 

 Morozzo has inferred, that the period of incubation is 

 proportioned to the duration of the life of the bird ; in 

 the same way as he conceives the gestation of mankind 

 and quadrupeds is proportioned to the length of their 

 existence. The swan, which is supposed to live 200 

 years, sits 42 days ; the goose, supposed to live 80, sits 

 30; while the goldfinch, whose life is restricted to 18 

 or 20 years, does not sit on its eggs above l<t days. 



The experiments of most observers have been made 

 on the eggs of the common hen ; but Malpighi did not 

 confine himself to these exclusively. On inspecting an 

 egg at the larger end, where all the changes take place, 

 several conspicuous parts appear, as the albumen, the 

 yolk, the air vessel, and the membranes, besides some 

 which are less obvious ; but the highest magnifier is in- 

 capable of discovering the germ of the chicken. No- 

 thing more than a confused whitish spot, generally call- 

 ed the cicatricitla, is discernible. Malpighi, indeed, 

 who has treated this subject in a masterly manner, 

 Mans to affirm, that the embryo is perceptible before 

 incului.'on, but it has been sensible to no other obser- 

 ver. I Ic likewise thought its evolution would be seen 

 in MX hours after incubation, and has given a magnified 

 delineation of the appearances. Haller considers it un- 

 necesory, at least in the climate of Switzerland, to 

 make observations previous to seven hours ; and lie re- 

 marks, that Malpighi'* egg of six hours was as far ad- 

 vanced as his of twenty-four. On the whole, it is to 

 be concluded, that none of the more evident changes 

 happen before the twelfth hour. At that time a slight 

 enlargement of the cicatrk-ula is found to have taken 

 place. The embryo, still of the most irregular and in- 

 di^tinct figure, has undergone some expansion : it is 

 surrounded by a line of a greenish or deep yellow co- 

 lour. The rings or halones environing it, of which 

 sometimes three and sometimes four are visible, seem 

 better defined. No alteration is yet perceptible on the 

 bulk or consistence of the fluids, though Haller says 

 that an egg, with seven hours incubation, contains a 

 greater quantity of albumon than it will ever have af- 

 terward*. A cavity at the large end, called thefollicu- 



ATION. 



Int atrit, is full of atmospheric air, which is gradually Incubation, 

 enlarged by incubation ; and although twenty-one eggs ^ 

 newly laid, afford only a culric inch of air, the. same 

 quantity is yielded by two after twenty dayv 



Considerable alteration will be found to have taken 

 place in twelve hours more, or after the interval of a 

 day, from the commencement of incubation. The egg 

 floats with the small end downwards, by the enlarge- 

 ment of the cavity, or escape of its own fluids, and 

 the figure of the embryo is better defined. It seems 

 about .18 of an inch in length, whereas in 12 hours 

 it does not exceed .1. The circles or halones are 

 more extended, and the interior one has acquired some 

 resemblance to the figure of a heart, surrounded by a 

 line of a greenish colour. Green specks are likewise 

 perceptible in different parts of the area enclosed by 

 it, which are the rudiments of blood vessels. Twelve 

 hours later, the greenish line has gained more evi- 

 dently the appearance of a vessel, and some of the 

 specks are seen to contain blood> They are distinct 

 blood vessels in two hours longer, and convey red blood 

 in six more. In 48 hours, the foetus betrays signs of 

 motion. Its head resembles a hammer, and the tail is 

 extremely slender. The vascular area has attained the 

 figure of a heart, and pulsation is now evident, or per- 

 haps some hours earlier. Still " it appeared like a 

 cloud, and melted under the scalpel." At this period 

 also the eyes become visible ; and the contents of the 

 yolk bag have grown somewhat more fluid. It was on- 

 ly at this time that Haller succeeded in measuring the 

 length of the embryo, which proved .22 of an inch in 

 one egg, and .30 in another. It must not be supposed, 

 that there is the most distant resemblance between this 

 shapeless body and a bird ; on the contrary, no two ob- 

 jects can be more dissimilar. At the end of 70 hours, 

 the rudiments of the wings, and sometimes those of 

 the legs, may be traced ; and at the close of the third 

 day, or 72 hours, the changes are still more conspicu- 

 ous, and the parts better defined. Before the fourth 

 has come to a conclusion, the pulsations of the heart 

 are so tremulously rapid, that the eye cannot follow 

 them. But as the life of the embryo is enfeebled by 

 exposure to the air, they become retarded, and are seen 

 to follow each other in regular succession. The liver 

 is perceptible before the fifth day, -not of its ultimate 

 hue and aspect indeed, but as a viscous cloud of a yel- 

 low colour ; and the eye also appears black ; the limbs 

 seem to move against its termination. The heart gains 

 its proper formation, and the viscera are unfolded. On 

 the sixth day life is so vigorous, that it is not uncom- 

 mon to find the chicken lying on its back, instead of 

 on its left side as before. The natural figure is in a 

 considerable degree acquired ; and most of the organs 

 exhibit a certain definite formation. The weight of 

 the egg has now greatly diminished, the external co- 

 lour darkens, and the substance of the shell becomes 

 more brittle. Next d;iy the white is smaller, and has ' 

 sunk towards the small end of the egg, while the di- ' 

 mensions of the yolk are enlarged. From this period, 

 the size of the animal advances with great rapidity. Its 

 members are gradually unfolded, and its strength pro- 

 gressively increased. The muscular nature of the sto- 

 mach is declared on the eleventh day. On the thir- 

 teenth the albumen is seen to have attained a greenish 

 hue. and to have acquired greater consistence; the 

 chicken has gained a more perfect form ; its head is 

 greatly curved, and the bill lies under the right wing : 

 It is pointed, and is sometimes opened and closed. On 

 the nineteenth, half the egg is occupied by air, and the 

 chicken is heard to cry. In the next place, the shell be- 



