In November of the same year Réaumur recommenced his experiments, and again exposed a number of pupæ to the genial influence of these hot-houses. The result was the same. In the first week in December butterflies appeared, which would not, in natural circumstances, have been developed earlier than the May of the next year. There were some pupæ, in particular, whose development he watched with great interest. These pupæ belonged to a beautiful species of moth, which has two broods in a year; that is, it lays eggs in May which become butterflies in July, and then again lays eggs which become pupæ in August or September, but do not become butterflies until the following June. He was curious to see whether this second brood, instead of waiting for several months, would, like the first, disclose its butterflies in a considerably shorter time, now that it was exposed to the warmth of the Conservatory. Such actually proved to be the case; and thus two generations of these butterflies were obtained in one year. Alluding to the depredations of caterpillars, he quaintly remarks, "This certainly is not a secret which appears very profitable at present; but who can tell whether that which is useless to us to-day, may not possibly become of value to-morrow? Could we discover some new species of larvæ which would supply us with as good a silk as that of the silk-worm, and might be more easy to rear, but which only produced one generation in each year, and if it lived upon leaves which could be found all the year through, we might avail ourselves of this means of increasing the number of its broods." This remark deserves much consideration. He was now anxious to try whether, by applying a more equable method of warming, he could succeed in hatching pupæ as he had done in the hothouse. The idea occurred to him of endeavouring to hatch them _under a hen_. He concluded that the warmth of the mother's breast would quite as easily hatch the insects, as it does the eggs. But there was this obstacle in the way: How could he prevent the fragile and tender bodies of the insect pupæ from being crushed and killed by the weight of the hen's body? and, as we would also suggest, How could he ensure that the bird would not actually have eaten up the objects of his care? Aware of the fact, that, when a hen is in the humour to sit, she will often allow smooth stones to be placed among her own eggs, he anticipated no difficulty on that score, and he hit upon the following ingenious experiment:--He procured some hollow glass balls which he had caused to be made as nearly as possible similar in size and shape to the eggs themselves. Into these, by an opening at one end, he introduced seven or eight pupæ, and stopped the mouth up with a cork, but so as to allow a free communication with the external air by paring off a piece from the side of the cork.