My dear P.M.,
Whilst in Vienna I heard several versions, varying only in detail, of the civil disturbance that occurred there this year, in which nearly one hundred people were killed mostly by rifle and machine gun fire turned on them by the police.  The story is of no great moment, other than that of being a particularly good example of how police can infuriate a stupid and rather aimless mob who want to 'demonstrate' and probably make a bit of trouble. One thing led to another in this case in perfect sequence until shooting started, and then became blind and indiscriminate. I have always thought that tear-gas bombs would be ideal for coping with this type of disturbance, and I see no reason why police authorities do not make provision for the simple training of their personnel in their use -keeping the actual bombs in store for issue in emergency, just as lethal weapons are now kept ready for issue. Rifle and machine gun fire on a mob seems to be an inhuman and unnecessary proceeding when other more suitable methods are available.
In this respect I have looked up C.I.D. Paper 760-B of January 1927, the essential part of which I reproduce below:-
It is for consideration whether the armed forces of the Crown in China should be supplied with tear gas for use against mobs, as being more humane, as effective and less likely to cause diplomatic difficulties than lethal weapons.
In the Washington Agreement and recent Geneva Protocol on the subject of gas, it is laid down that gas is not to be used in war;
tear gas is included in this prohibition.
That this prohibition does not apply in peace time is apparently the view of certain nations, as authentic information of its use by France and the United States in quelling civil disturbances has been received.
This attitude is only logical, for it is far more humane and, under most circumstances, just as effective as the use of lethal weapons.
The tear gas used by us during the war is a liquid known as S.K., the vapour from which produces the effect. It is not suitable for use against mobs as, being a liquid, its effect may persist in the locality where it is applied for hours and even days, also a splash of the liquid in the eye may cause permanent blindness. A more suitable tear gas is one known as C.A.P., which is non- persistent and cannot cause any permanent injury. Unfortunately the only stocks of tear gas in this country are of S.K., and no facilities exist for the immediate production of C.A.P.
This latter gas, and suitable means for using it in the form of small cartridges in police batons and small hand grenades, could, if desired, be purchased on the open market in America.
Possible retaliation by the Cantonese  with more lethal gases than tear gas must be considered. With regard to this, intelligence reports indicate that they have a laboratory engaged in poison gas research in the Whampoa Military School , but that their stocks of gas are small. They also are known to have been supplied with a few hundred Russian gas masks. On the other hand, if they are permitted to receive unrestricted supplies of munitions from Russia, that country could supply them with gas in quantity.
Our gas mask, which has been supplied to all the naval personnel, is proof against all gases that could be used by the Chinese.
Should supply of tear gas be agreed to, it is suggested that very definite instructions as to the circumstances under which it should be used should be given.
Yours sincerely, R. G. CASEY