Slide showing two line graphs each with four lines which concern a chemical equation Ng2HPO4/100 cc[s] Medium. Slide shows two line graphs each with four lines which concern a chemical equation Ng2HPO4/100 cc[s] Medium. Each line is marked with its own symbol. There is a symbol for control and the other three symbols vary depending on which graph they appear on. X-axis is labelled 'Days' with a scale from 2 - 12. Y-axis is labelled 'Exhibition' with a scale from 0.70 - 1.00. Late 19th to early 20th century