Lazzaro Spallanzani modified the Needham experiment in 1768, attempting to exclude the possibility of introducing a contaminating factor between boiling and sealing. His technique involved boiling the broth in a sealed container with the air partially evacuated to prevent explosions. Although he did not see growth, the exclusion of air left the question of whether air was an essential factor in spontaneous generation. However, by that time there was already widespread scepticism among major scientists, to the principle of spontaneous generation. Observation was increasingly demonstrating that whenever there was sufficiently careful investigation of mechanisms of biological reproduction, it was plain that processes involved basing of new structures on existing complex structures, rather from chaotic muds or dead materials. Joseph Priestley, after he had fled to America and not long before his death, wrote a letter that was read to the American Philosophical Society in 1803. It said in part: