It is well known that at the beginning of the churning process a considerable volume of gas is evolved, necessitating some method of ventilation of the churn. In old-fashioned churns the gas was allowed to escape by simply withdrawing the cork or plug, while all modern churns have a special arrangement for the purpose. The gas set free has been stated to be carbon-dioxide, but hitherto no definite proof of its source or estimation of its amount has been recorded. It cannot very well be that the gas is formed by any chemical reaction at the time the churning process is going on. It seems more probable that it is produced by bacterial action during the ripening of the cream, held in a state of super-saturation by the liquid and released by the shaking in the churn in the presence of air. This view would account for the fact that it is only during the first hundred or so revolutions of the churn that any quantity of gas is evolved. It was with the object of demonstrating the correctness or otherwise of this latter theory that the following experiments were carried out.