556 Retrospective Criticism. 



to insects than oil of turpentine, is also very efficacious ; when 

 it is pure, its deposit is less considerable, but still may be suf- 

 ficient to render the adoption of it dangerous. Oil of cloves 

 is nearly free from this objection, but, notwithstanding its ex- 

 ceedingly strong odour, its destructive effects are compara- 

 tively feeble. Oil of thyme (oleum serpylli of the shops) is 

 far more fatal than even oil of turpentine, and, perhaps, leaves 

 less deposit than almost any other essential oil : it has, how- 

 ever, this powerful objection, that the cabinetmaker's art will 

 scarcely enable him to construct drawers sufficiently tight to 

 prevent the apartment being impregnated with its detestable 

 odour. Many French entomologists, who denounce spirits of 

 wine as unfit for any insects except the black Coleoptera, put 

 into their collecting bottles coils of paper imbued with some 

 drops of this oil ; and on crossing the path of one to leeward, 

 on a good scenting day, you may nose him two fields off. A 

 good way to judge of these oils is to suspend in small wide- 

 mouthed phials pieces of sponge, of equal size, each moistened 

 with precisely the same quantity of oil, and after leaving them 

 in a warm place for about a fortnight, the finger applied to 

 the inside surfaces of the phials will readily detect the differ- 

 ence of the films deposited. Poisoning the individuals, in the 

 manner described by Mr. Waterton, affords, no doubt, the 

 most perfect security, but the operation requires time, which 

 a great proportion of the lovers of the science may not have 

 at their command. Godart proposes for lepidopterous insects 

 the application of arsenical soap, a preparation which no one 

 who can avoid it would be desirous to meddle with ; he also 

 recommends that individuals obtained by purchase or ex- 

 change, should perform quarantine in a tin lazaretto, with a 

 little oil of petroleum. Although this attack on their olfactories 

 might force the larvae and beetles to break cover, and probably 

 kill the pupae, would it destroy the vitality of their eggs also ? 

 if not, the precaution would afford but temporary security. 

 Notwithstanding the denunciations of my French friends, I 

 have found spirits of wine, when properly diluted, unobjec- 

 tionable for the collecting bottle, with some exceptions, which 

 a short experience will point out (the insects which suffer most 

 are all red and many delicate blue ones, and such as are co- 

 vered with a glaucous powder) ; and in autumn, 1831, I re- 

 solved to try the effect of adding the smallest quantity of 

 corrosive sublimate, which I thought might be sufficient to 

 preserve the specimens immersed in the solution from the 

 future attacks of insects. I had strong apprehensions on the 

 subject of their flexibility and colours. My first experiments 

 were, of course, comparative, and confined to common species ; 



