Reid's Elements of Practical Chemistry, 175 



rents are brought about by a power which the insect possesses of inflating 

 its body and head. This last organ, ultimately destined to become fixed 

 and solid, is at this period capable of contraction and dilatation, like a 

 membrane. 



" The head and corslet being exposed, the legs are drawn out from their 

 nymphine cases. At this period every part of the insect is soft. After 

 having protruded itself thus far, it hangs with its head downwards, and 

 remains motionless, so as to lead the observer to believe that the eiforts 

 which it had hitherto made had exhausted its strength, and that it had 

 thus perished in the act of being born. However, it remains in this posi- 

 tion just so long as to permit its body and limbs to be hardened and dried 

 by the air, and then reverses it, forming an arch ; this enables the insect 

 to draw out its tail from the mask. 



" When it has just cast off that tenement in which it had till now existed, 

 the body of the libellula is soft, has not attained its full length, and the 

 wings are still folded. It remains, therefore, tranquil and motionless till 

 these important operations have taken place, which are finished, sooner or 

 later, according to the heat or moisture of the atmosphere. The operation 

 may be completed in a quarter of an hour, or take up several hours, ac- 

 cording to circumstances. The wings unfold themselves in every direc- 

 tion ; — it is supposed that this curious mechanical effect is brought about 

 by means of the fluids, which rush into and distend them ; for they remain 

 drooping as wet paper if the insect die in the act of metamorphosis ; so 

 that something more than drying is necessary. During the time that the 

 wings, from being shrivelled and flexible, are becoming firm and glistening 

 as talc, the dragon-fly takes care not to allow even its own body to obstruct 

 their expansion in the proper direction, and for this purpose bends it from 

 them ; for if they took a wrong fold at this moment, they would for ever 

 retain the deformity. Provision is even made to prevent the wings from 

 coming in contact with each other ; for, instead of being all in the same 

 horizontal plane, as they subsequently are, they are perpendicular to the 

 insect, and thus ranged side by side." 



We have only to add, that the volume is got up, like all the other 

 volumes of this popular Family Library, in a style of great neatness, highly 

 creditable to the pi\blisher, and that the wooden cuts by which the subject 

 is illustrated, possess all the sharpness of copper engravings. 



III. Elements of Practical Chemistry, comprising a series of experiments 

 in every department of Chemist?^, with directions for performing them, 

 and for the preparation and application of the most important tests and 

 re-ugents. By David Boswell Retd, Experimental Assistant to Pro- 

 fessor Hope, Conductor of the Classes of Practical Chemistry in the Uni- 

 versity of Edinburgh, &c. &c. &c. 1830, 1 vol. Svo. 662 pp. 



The author of the present work has been advantageously known to the 

 public by an excellent popular Treatise on Chemistry, in 2 vols. 12mo, and 

 by a pamphlet explanatory of his improved scale of chemical equivalents, 

 in which hydrogen is taken as a standard of comparison. Since the pub- 



