I HAVE read with much interest Sir Ray Lankester's letter on the colour of water in NATURE of March 17. I remember discussing this same problem in the case of ice with Sir Joseph Thomson when we stood at the foot of the great glacier at Glacier, B.C., during the western excursion of the British Association last summer. The rich blue colour of the hard, clear ice was remarkable, even in quite small pieces. The same blue colour is noticed When surface-ice, which has been formed slowly by conduction, is taken out of the St. Lawrence River. The blocks lose their colour when they are exposed for long to the light, and especially rapidly when exposed to sunlight. Coloured sediment and air cavities in the ice detract from the colour. I am inclined to believe that the colour of ice is a real absorption effect, due to the large molecular aggregates forming the structure, which absorb the long rays, and not a “blue sky” effect, as I suggested after seeing the blue ice of the glacier.