54 



Prof. W. C. Unwin. 



[Nov. 18 r 



goes hand in hand with the chemical changes which occur. From the 

 spectroscopic appearance it is argued that the bine precipitated oxide 

 is not a hydrate, but that it does very readily undergo change as the 

 mere alteration of colour which takes place shows. Aqueous solutions 

 of the bromide and iodide of cobalt when acted on by alkalies undergo 

 changes similar to those which the chloride undergoes, and, as in the 

 former cases, the iodide spectrum is always nearer the red end of the 

 spectrum than the corresponding bromide spectrum, and the bromide- 

 than the chloride spectrum. 



The salts of the oxygen acids when in aqueous solution do not give 

 sharp banded spectra as the haloid salts do, but only a large shading 

 off absorption like the hydrate of the cobalt chloride. 



The other points discussed in detail are, first, the nature of the 

 precipitate formed by the action of sodic or potassic carbonate on a 

 cobaltous salt, and it is shown that the formation of the oxide always 

 found in this precipitate is owing to an after decomposition, the pre- 

 cipitate as first formed being entirely free from all oxide, and it 

 gradually appearing after a short time. The other point is the action 

 of heat on cobalt phosphate dissolved in fused microcosmic salt ; when 

 cold there appears somewhat indistinctly a banded spectrum of a 

 phosphate, on heating this the spectrum disappears, and the spectrum 

 of the oxide very distinctly takes it place ; on cooling, the first 

 spectrum returns, and this change may apparently be repeated any 

 number of times. 



Drawings of all the different spectra are given in the full paper. 



I V. " On the Friction of Water against Solid Surfaces of Diffe- 

 rent Degrees of Roughness.*' By Professor W. C. Unwin, 

 M.I.C.E., Professor of Hydraulic Engineering at the Royal 

 Indian Engineering College. Communicated by J. H. 

 Cotterill, F.R.S., Professor of Applied Mechanics, Royal 

 Naval College, Greenwich. Received August 31, 1880. 



(Abstract.) 



These experiments relate to the friction of fluids when flowing 

 against rough solid surfaces. It is well known that a board dragged 

 through water suffers a resistance which, at speeds not very small, 

 varies nearly as the square of the velocity. The fluid surrounding 

 the board does not behave as a solid, but shearing and eddying motions 

 are set up which give rise to losses of energy distributed throughout 

 the fluid mass. 



Most of the existing knowledge of fluid friction has been derived 

 from the observation of the flow of water in pipes and canals. But irt 



