New University Building
As the University's insatiable growth continued, a new building was required to replace the aging Old Main University Building. The cornerstone of New Main Building was laid September 13, 1871, and it was dedicated two years later on Wednesday, December 10, 1873, with Governor J.L. Beveridge giving the address.  A Victorian and Second Empire Style building with a French mansard roof, it was designed by John Mills Van Osdel, who is widely recognized as the first professional architect in the Western United States, and was also an original member of the Board of Trustees from its charter in 1867 to 1873.  It also housed the University Chapel, where mandatory services were held until 1891, when student opposition culminated in protests that led to their elimination. 
The new building was 214 feet long by 112 feet high with twin wings that reached 124  feet deep and towers stretching to 145 feet tall. The building cost $148,000 to build, not including furnishings and the heating system. It had over 60 large rooms "devoted to public uses, as Class Rooms, Lecture Halls, Library and Reading Rooms, Museums, Draughting Rooms, Laboratories, [and] Society Halls".  Five rooms alone were dedicated to the library and museums.  The building had a capacity of over 1,000 students , a sign of the University's explosive growth from the three students who had enrolled its opening day just five years earlier. 
In January of 1938, a classroom ceiling collapsed, forcing the building to be razed.  However its front entranceway and several other artifacts were preserved. The portal was kept in the courtyard of the Architecture building for many years until eventually making its way to Robert Allerton Park in Monticello, where it was rediscovered in 1994. Alan and Phyllis Welsh Hallene donated funds to build a plaza on modern Lincoln Avenue in 1998 that features the original portal.
Despite the building having been razed nearly 70 years ago, one still has the opportunity to take a trip through time to see what things once looked like when the building anchored the UI campus. In 1873, when the building was completed, the University of Arkansas at Fayetteville purchased the plans for the building and erected an identical building, finished in 1875, that still stands on their campus as Old Main.  They even attempted to recreate the surroundings of the UI's New Main Building, and a visitor to their campus today will feel as if they have traveled back to the early days of the Illinois Industrial University's campus. It was recently renovated in 1991 and currently serves as a primary classroom building on their campus.
While popular belief holds that the Illini Union was built on the exact site of the former University Hall, in fact, University Hall was much farther south, with its northern section on the same line as the southern section of Altgeld Hall. Essentially its northern tip was where the southern tip of the Union is.