318 Transactions of the South African Philosophical Society. 



tissues and such which secrete them by special organs on their 

 surface. In the former case, for which the orange and lemon, the 

 parsley and celery, the laurel and the eucalyptus are well-known 

 examples, there is hardly any difference of opinion with regard to 

 the protective value of these oils, although authors, when dealing 

 with the second group of plants, have not always been careful 

 enough to discriminate properly between them. 



Of special importance in connection with this question are the 

 experiments on snails and slugs by Stahl.* He proved, fairly 

 conclusively, that many plants which were not injured by snails, 

 although the latter had no other food, were devoured by them 

 eagerly when the essential oils had been extracted by means of 

 alcohol or by drying. In the latter case the leaves had to be soaked 

 in water before being offered to the snails. Stahl extended his 

 experiments also to plants of the second group with identical results, 

 but here his conclusions have not been accepted so generally, owing 

 to another very ingenious explanation of the function of these oils. 

 Plants belonging to this group are particularly numerous in the 

 order Labiatae ; the thyme, peppermint, sage, rosemary, and our 

 own wild dagga (Leonotis Leonurus, E.Br.) being familiar examples 

 of this order. 



This other theory is based upon the famous experiments of John 

 Tyndall with regard to the influence which the vapours of essential 

 oils have on the diathermancy of the air in which they occur. 

 Tyndall f found that when heat-rays have to pass through such a 

 mixture, a much larger proportion of them is absorbed than when 

 they are passing through air alone, and that consequently an object 

 enveloped by such a mixture of air and ethereal oil vapours would be 

 less heated than when surrounded by air only. He filled a tube 

 with a dried aromatic herb, such as peppermint, passed a slow 

 current of air through the tube and examined this mixture of air and 

 vapour in another tube with regard to its permeability for heat-rays. 

 He found that air with the vapour of lavender gave an absorption 

 of heat 32 times as great as that of air alone, thyme 33, peppermint 

 34, and wormwood 41 times. 



When he employed the essential oils instead of the herbs by 

 placing bibulous paper saturated with the oil into his first tube, he 

 obtained much larger figures, for lavender oil gave 60, thyme 74, and 

 oil of aniseed even 372. 



When these observations became known to biologists, they 



* Stahl, B., " Pflanzen und Schnecken," Jena, 1888. 



f Tyndall, John, " Heat Considered as a Mode of Motion," London, 1863, 

 p. 360. 



