I n the Bakerian Lecture for 1894 (' Phil. Trans.,' vol. 185 A, p. 397), we gave an account of our work on the viscosity of some seventy liquids at different tempera tures between 0° and the ordinary boiling-point, and we discussed the inter dependence of viscosity and chemical composition. Among the liquids dealt with on that occasion, there was no member of the important series of esters or ethereal salts, and, further, only one ether-ordinary ether or ethyl oxide-was included in the list. We

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... therefore thought it desirable, in order to make the investigation more complete, to obtain data for members of these two classes of organic substances. The physico-chemical relationships previously established made such determinations of particular interest. Among the various connections traced between chemical constitution and viscosity, one of the most striking was the different effect which oxygen exerted upon viscosity according to the different modes in which it was assumed to be associated with other atoms in the molecule. The influence which could be ascribed to hydroxyl-oxygen differs to a most marked extent from that of carbonyl-oxygen, and, although only three cases were studied, it appeared that ether oxygen, or oxygen linked to two carbon atoms, had also a value which differed considerably from those of oxygen in other conditions. There was thus an additional reason for making observations on esters and ethers, since both contain ether-oxygen. In what follows we give the experimental values for the ten lowest fatty esters, carefully purified samples of which had been kindly placed at our disposal by Professor Sydney Young, F.R.S., to whom we desire to tender our thanks. At our suggestion Mr. R. E. Barnett, B.Sc., Assoc. R.C.S., prepared five fatty ethers, and determined their viscosity. An Appendix to this paper is devoted to an account by him of the best methods of obtaining these liquids. As we had the opportunity of making determinations of the viscosity of three different samples of isopentane and a sample of ethyl-benzene, which had been specially prepared and purified, the values obtained are also included in this paper. The last section of the paper deals with the discussion of the values given by the esters and ethers according to the methods adopted in our previous communication.