TAR. 337 



berg and Caruso an emulsion of 1 per cent benzene and 1 per cent soft 

 soap is the best way to destroy the tiger beetle and the least injurious 

 to the pear-tree. 



Pulvmaria Vitis, Coccus Vitis (vine scale, vine cochineal), Targioni- 

 Tozzetti (phylloxei'a of the vine). — Benzene destroys all the phylloxera 

 on the roots of the vine in pot, but has proved itself inefficient on the 

 large scale according to Mouillefert. A stock stripped received ^ litre 

 of benzene emulsified in 10 litres of water ; the greater part of the phyl- 

 loxera were destroyed it is true, but the roots at a great depth still bore 

 numerous living lice. Benzene cannot therefore, it would appear, 

 replace carbon disulphide. 



126. Tar. — Tar is the liquid product which results from the dry 

 distillation of coal and numerous organic bodies. Its nature varies 

 with the raw material which gave it birth, but it is always oily, viscous, 

 of a dark colour, and empyreumatic odour. Tar is insoluble in water. 



1. Coal Tar. — Coal tar is a by-product of gas manufacture. It 

 consists of a mixture of some fifty organic compounds, amongst which 

 are benzene, toluene, naphthalene, and phenols. It is a black viscous 

 liquid. 



2. Wood Tar. — Wood tar is obtained by the dry distillation of wood. 

 It is a by-product in the manufacture of pyroligneous acid and wood 

 charcoal, also in the distillation of pines after the extraction of the 

 turpentine. When it is desired to obtain tar chiefly, the distillation is 

 slackened and 18 per cent of tar may be obtained in that way. The 

 nature of the tar varies with the nature of the wood and the method of 

 extraction used. Tar obtained by carbonizing wood has neither the 

 same, composition nor the same appearance as that obtained by distilla- 

 tion. The first is black and approaches coal tar ; it contains a large 

 amount of naphthalene. The second is pale, almost viscous, and con- 

 tains paraffin. The composition of wood tar is thus more complex 

 than that of coal tar. It contains benzene, toluene, styrolene, naphtha- 

 lene, phenols, guaiacols, creosote, and other aromatic compounds. Tar 

 is used in human therapeutics as an antiparasitic agent. 



Action of Tar on Plants. — Tar is a substance comparatively 

 harmless to the tree. Used such as it is on restricted surfaces of the 

 bark of a tree — for example, as a circular ring to prevent the access of 

 insects or to cover wounds — it is without action on the growth and does 

 not injure the tree.^ Tar does not kill the cells which it penetrates, but 

 it preserves them from decomposition so that neighbouring cells may 

 grow and develop normally. The cellular layer which is penetrated by 

 the tar only measures 1-2 millimetres in summer. It is therefore 

 better to apply tar to wounds in winter. It has been found that tar, 

 after remaining seventy days on the tree had not been entrained by the 

 sap, and had not penetrated further than at the time of application. 

 However, Sorauer and Von Tuboeuf remarked that tar must not be 

 abused on fresh w^ounds or on young trees, for in their opinion it is 

 only old wounds and old trees which stand tar with impunity. Eobert 



1 Translator's Note.— This is no doubt true as regards old trees, but tar should 

 not be applied to young trees with tender bark. 



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