THUNDER and lightning have been much maligned over the years. The Venerable Bede, that noted man of science of the Dark Ages, firmly believed, for example, that thunder with a west wind produced the "fumosites" which presaged "a very bad pestilence". And until comparatively recently there was a widespread belief that milk turned sour when there was thunder in the air.
The idea that the souring of milk in thundery conditions might have something to do with electricity was scotched as long ago as 1913 by two scientists called Mr Duffield and Mr Murray, who showed in a series of experiments that atmospheric electricity, far from turning milk sour, might even act as a preservative. They drew in air through a tube in which were fixed two live electrodes; a high voltage was applied to the electrodes, and the air "electrified" by the resulting discharge was then allowed to "bubble" through a flask of milk. The experiments suggested that the acidity - the tendency to sourness - of "electrified" milk increased much more slowly than that of the ordinaire variety over a given period; in fact if the strength of the electrical discharge was increased beyond a certain point, the level of acidity of the treated milk increased hardly at all, even after about four days.
The souring of milk, or the production of tactic acid, is caused by the action of bacteria - called, appropriately enough, bacilli acidi lactici. These bacteria are comparatively inactive at temperatures below 7 degrees Celsius, but their multiplication becomes increasingly rapid at progressively higher temperatures, up to somewhere in the region of 38 degrees C. And this explains the association of thunderstorms with sour milk.
The tendency to sourness is caused, not by the thunder itself, but by the coincidence of the warm humid conditions which usually precede a summer thunderstorm. It is the favourable temperature, rather than electricity in the air, which turns the milk sour - an explanation which is strengthened by the relative infrequency of the problem in the case of winter thunder-storms. It is also noteworthy that one of the traditional ways of preserving milk was to place the container of milk in a shallow basin of water, into which a muslin cloth covering the jug was allowed to drape; the water used the muslin like a wick, and its evaporation from the cloth resulted in a drop in temperature. But this method of keeping the milk fresh works only when the air is relatively dry, allowing copious evaporation - and would be virtually ineffective in the humid conditions typical of summer thundery weather.