143 



diverge before they reach the object ; and it may be remarked gene- 

 rally that the definition of objects is always most perfect, when an 

 illuminating pencil of suitable form is accurately adjusted to focus, 

 that is, so that the source of light and the plane of vision may be 

 conjugate foci of the illuminator. If an object-glass of 120 aper- 

 ture or upwards be used as an illuminator, the markings of Diato- 

 macese will be scarcely distinguishable, with any object-glass ; the 

 glare of the central rays overpowering the effects of structure on 

 those that are more oblique. 



XVI. "On the Constitution of Coal-tar Creosote." By Pro- 

 fessor WILLIAMSON. Communicated by Dr. SHARPEY, 

 Sec. E.S. Received June 15, 1854. 



For some years past it has been a debated question among che- 

 mists, whether the peculiar body originally described by Reichen- 

 bach as creosote, and subsequently analysed by Ettling and others, 

 has any real existence, or whether the properties which were attri- 

 buted to it are not to be more correctly ascribed to the hydrate of 

 phenyl, which can be obtained in a state of great purity from at 

 least one sort of commercial creosote by mere distillation, and which 

 possesses in an eminent degree the antiseptic properties for which 

 creosote is remarkable. 



With a view of obtaining some light on this question, Mr. Fairlie 

 undertook, in the laboratory of University College, an investigation 

 of the portions of coal-tar creosote which boil higher than the hydrate 

 of phenyl. The result of his experiments has been to show that a 

 body homologous to hydrate of phenyl may be obtained from the 

 crude creosote, in fact the next term of the series above hydrate of 

 phenyl itself. Some qualities of commercial creosote contain a 

 greater quantity of this hydrate of cresyl (as it may be termed) than 

 others ; and it is most advantageously prepared from those portions 

 which in the first distillation come over between 200 Cent, and 220. 

 After a great number of fractional distillations, a colourless, highly 

 dispersive liquid is obtained, boiling at 203 Cent., and possessing 

 the composition represented by the formula C H H 8 O 2 . 



