The Chamalières brewery was no longer sufficient for Pasteur; he wished to see one of those great English breweries which produce in one year more than 100,000 hectolitres of beer. The great French _savant_ was most courteously received by the managers of one of the most important breweries in London, who offered to show him round the works where 250 men were employed. But Pasteur asked for a little of the barm of the porter which was flowing into a trough from the cask. He examined that yeast with a microscope, and soon recognized a noxious ferment which he drew on a piece of paper and showed to the bystanders, saying, “This porter must leave much to be desired,” to the astonished managers, who had not expected this sudden criticism. Pasteur added that surely the defect must have been betrayed by a bad taste, perhaps already complained of by some customers. Thereupon the managers owned that that very morning some fresh yeast had had to be procured from another brewery. Pasteur asked to see the new yeast, and found it incomparably purer, but such was not the case with the barm of the other products then in fermentation--_ale_ and _pale ale_. By degrees, samples of every kind of beer on the premises were brought to Pasteur and put under the microscope. He detected marked beginnings of disease in some, in others merely a trace, but a threatening one. The various foremen were sent for; this scientific visit seemed like a police inquiry. The owner of the brewery, who had been fetched, was obliged to register, one after another, these experimental demonstrations. It was only human to show a little surprise, perhaps a little impatience of wounded feeling. But it was impossible to mistake the authority of the French scientist’s words: “Every marked alteration in the quality of the beer coincides with the development of micro-organisms foreign to the nature of true beer yeast.” It would have been interesting to a psychologist to study in the expression of Pasteur’s hearers those shades of curiosity, doubt, and approbation, which ended in the thoroughly English conclusion that there was profit to be made out of this object lesson. Pasteur afterwards remembered with a smile the answers he received, rather vague at first, then clearer, and, finally--interest and confidence now obtained--the confession that there was in a corner of the brewery a quantity of spoilt beer, which had gone wrong only a fortnight after it was made, and was not drinkable. “I examined it with a microscope,” said Pasteur, “and could not at first detect any ferments of disease; but guessing that it might have become clear through a long rest, the ferments now inert having dropped to the bottom of the reservoirs, I examined the deposit at the bottom of the reservoirs. It was entirely composed of filaments of disease unmixed with the least globule of alcoholic yeast. The complementary fermentation of that beer had therefore been exclusively a morbid fermentation.”