356 Lord Rayleigh. On the relative [Feb. 9, 



for it seems probable that such large tooth-rudiments may be traceable 

 in later stages. 



In this investigation, the few points of uncertainty as to the minute 

 structure of some of the tissues will in all probability be settled 

 satisfactorily.* 



DESCRIPTION OF FIGTJKE. x 40. 

 The arrow points towards the middle line of the mouth. 

 M. Mouth. 

 s, e. Superficial epithelium of oral cavity. 

 m. Sub-epithelial tissues forming tooth-sac immediately outside enamel-cap. 



a. Outer membrane of enamel-cap. 



b. Middle membrane of enamel-cap. 



c. Inner membrane of enamel-cap ; the enamel cells. The dark layer just 



external to the enamel cells represents the stratum intermedium of 

 Hannover. 



d. The dentine, of which the inner part stains faintly and shows the delicate 



dentinal tubes. 



s. Space caused by shrinkage between enamel cells and the outer part of the 



tooth rudiment. 

 p. Tooth papilla. 



o. The odontoblasts forming the superficial layer of the papilla. 



III. " On the Relative Densities of Hydrogen and Oxygen. 

 Preliminary Notice." By Lord Rayleigh, Sec. R.S., Pro- 

 fessor of Natural Philosophy in the Royal Institution. 

 Received February 2, 1888. 



The appearance of Professor Cooke's important memoir upon the 

 atomic weights of hydrogen and oxygenf induces me to communicate to 

 the Royal Society a notice of the results that I have obtained with respect 

 to the relative densities of these gases. My motive for undertaking 

 this investigation, planned in 18824 was the same as that which 

 animated Professor Cooke, namely, the desire to examine whether the 

 relative atomic weights of the two bodies really deviated from the simple 

 ratio 1 : 16, demanded by Prout's Law. For this purpose a knowledge 

 of the densities is not of itself sufficient ; but it appeared to me that 

 the other factor involved, viz., the relative atomic volumes of the two 

 gases, could be measured with great accuracy by eudiometric methods, 

 and I was aware that Mr. Scott had in view a redetermination of this 

 number, since in great part carried out.§ If both investigations are 



* This appears to have been already the case. — Feb. 6, 1888. 

 f " The Relative Values of the Atomic Weights of Hydrogen and Oxygen," by 

 J. P. Cooke and T. W. Richards, 1 Amer. Acad. Proc.,' vol. 23, 1887. 

 X Address to Section A, British Association ' Report,' 1882. 



§ "On the Composition of "Water by Yolume," by A. Scott, 1 Roy. Soc. Proc.,' 

 June 16, 1887 (vol. 42, p. 396). 



