All experiments concentrated upon certainty in sterilizing the substance operated upon. Pasteur pointed out that the chemical properties of the infusion affected the vitality of the microscopic germs inclosed in it; and Roberts at a later stage confirmed this by independent investigation, proving "that slightly alkaline liquids are more difficult to sterilize by heat than slightly acid liquids."[J] In this way, it was shown that distinct records of temperature were needful, greater intensity of heat being required in some cases than in others, in order to secure destruction of germs. The facts were illustrated by hay infusion, "the acid infusion invariably remaining barren after a few minutes' boiling, and the neutralized infusion invariably becoming fertile after a similar boiling."[K] The neutralizing element was liquor potassæ, and the next question started was this, Did the liquor potassæ enable the germs to live longer under the boiling process, or did its infusion operate so as to originate life where germs no longer had any existence? A contrivance was adopted by which the boiling could be applied to the hay infusion, while the liquor potassæ was kept enclosed in another part of the tube, ready to be added without exposure to the air, after the boiling process was over. When added in this way, "the liquor potassæ had not any power to excite germination:" the expectation that a certain mixture of acid and alkali would originate life was disappointed; all the earlier experiments were discredited. Still, some clung to their expressed belief, for there is a prejudice of advanced thought, as there is a prejudice of old beliefs. Tenacity of avowed opinion, with strong love of research, prolonged the inquiry, and led to more decided evidence.