Phenomena involving the tensile strength of water have been studied by a kinetic method—high speed motion photography of the rapid movement of a blunt glass rod (5 mm diameter) in a narrow (16 mm inside diameter) glass tube of water. Special precautions have been taken to remove all hydrophobic patches and small gas masses (gas nuclei) but to retain the dissolved gas (air at one atmosphere) in the water. If the rod surface contained gas nuclei, or was hydrophobic and free of gas nuclei, cavitation occurred at the rear end when the velocity was less than 3 meters/sec., but if completely hydrophilic and free of gas nuclei, the velocity could be 37 meters/sec. or 83 miles/hour without cavitation. Addition of a detergent (diactyl sodium succinate) to the water did not prevent cavitation at a low velocity with the hydrophobic rod free of gas nuclei. Movement of a rod in pure corn syrup (viscosity 20.1 poises), free of gas nuclei, left a large cylindrical cavity that collapsed in a matter of hundredths of a second. It is not possible to calculate the tensions developed in these experiments, but it is pointed out that the velocities attained without cavitation are far higher than previously observed for movement of bodies in an aqueous medium, a result believed to be owing to the absence of all gas phases and hydrophobic surfaces.

I n a previous paper ( '44) we have considered the various conditions under which bubbles appear in liquids, the existence and formation of gas forming nuclei and the physical factors involved in their stability and growth, with illustrations from simple models. Physiological conditions and body changes which may be expected to affect bubble formation have been briefly enumerated. The technique for removing gas nuclei and their distribution in body fluids and tissues will now be described.RENOVAL OF GAS NUCLEI I n testing the blood of animals for gas nuclei or in any high gas supersaturation experiments, it is necessary to use containers freed of gas films and minute gas masses. Only then is it certain that bubbles are not introduced into the liquid under investigation and only then will the solution remain supersaturated. The necessities are a surface completely in contact with water and a liquid free of dust particles on which air pockets may persist. Such conditions are partially realized by thoroughly cleaning glass surfaces (as by cone. H2S04 and bichromate) and by strong centrifuging of the fluid under investigation. We have also used a filter to remove dust but centrifuging is preferable since filtering, unless very carefully done, may introduce more gas nuclei than it removes. A centrifugal force of 4000 X gravity for 15 to 30 minutes will remove gas nuclei from the walls of clean hydrophilic vessels as well as suspended particles. Such water will not spontaneously bubble when evacuated to its vapor pressure but does break

from 10-20 cc. of absolute ethanol gave 1.6 g. melting 91-92°; 70% of the theoretical yield. No depression was observed in a mixed m. p. with the product obtained before.

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